pktbuf: clarify doc of pktbuf_add()
pktbuf: clarify doc of pktbuf_add()

Fixes #2853

alex1818/RIOT,alignan/RIOT,asanka-code/RIOT,basilfx/RIOT,biboc/RIOT,Josar/RIOT,jremmert-phytec-iot/RIOT,altairpearl/RIOT,tdautc19841202/RIOT,RIOT-OS/RIOT,dhruvvyas90/RIOT,dailab/RIOT,Darredevil/RIOT,kYc0o/RIOT,khhhh/RIOT,lebrush/RIOT,gebart/RIOT,DipSwitch/RIOT,ThanhVic/RIOT,jbeyerstedt/RIOT-OTA-update,dailab/RIOT,kushalsingh007/RIOT,ant9000/RIOT,daniel-k/RIOT,miri64/RIOT,haoyangyu/RIOT,l3nko/RIOT,kYc0o/RIOT,phiros/RIOT,FrancescoErmini/RIOT,l3nko/RIOT,PSHIVANI/Riot-Code,herrfz/RIOT,mziegert/RIOT,rfswarm/RIOT,yogo1212/RIOT,thiagohd/RIOT,katezilla/RIOT,rfswarm/RIOT,PSHIVANI/Riot-Code,dailab/RIOT,haoyangyu/RIOT,alignan/RIOT,backenklee/RIOT,gebart/RIOT,tfar/RIOT,gebart/RIOT,x3ro/RIOT,RBartz/RIOT,jremmert-phytec-iot/RIOT,lazytech-org/RIOT,gbarnett/RIOT,TobiasFredersdorf/RIOT,msolters/RIOT,miri64/RIOT,luciotorre/RIOT,abkam07/RIOT,OTAkeys/RIOT,patkan/RIOT,shady33/RIOT,centurysys/RIOT,biboc/RIOT,robixnai/RIOT,JensErdmann/RIOT,jfischer-phytec-iot/RIOT,rakendrathapa/RIOT,lazytech-org/RIOT,lebrush/RIOT,l3nko/RIOT,arvindpdmn/RIOT,Lexandro92/RIOT-CoAP,rfuentess/RIOT,chris-wood/RIOT,rfswarm/RIOT,kaleb-himes/RIOT,RIOT-OS/RIOT,aeneby/RIOT,smlng/RIOT,plushvoxel/RIOT,alex1818/RIOT,kushalsingh007/RIOT,binarylemon/RIOT,beurdouche/RIOT,koenning/RIOT,herrfz/RIOT-old,sgso/RIOT,MarkXYang/RIOT,automote/RIOT,katezilla/RIOT,latsku/RIOT,phiros/RIOT,aeneby/RIOT,lazytech-org/RIOT,gautric/RIOT,yogo1212/RIOT,josephnoir/RIOT,Yonezawa-T2/RIOT,kerneltask/RIOT,tfar/RIOT,OTAkeys/RIOT,rajma996/RIOT,jferreir/RIOT,abkam07/RIOT,rousselk/RIOT,hamilton-mote/RIOT-OS,LudwigKnuepfer/RIOT,robixnai/RIOT,backenklee/RIOT,kushalsingh007/RIOT,neumodisch/RIOT,tfar/RIOT,sgso/RIOT,EmuxEvans/RIOT,abp719/RIOT,lebrush/RIOT,mfrey/RIOT,kb2ma/RIOT,daniel-k/RIOT,bartfaizoltan/RIOT,LudwigOrtmann/RIOT,haoyangyu/RIOT,smlng/RIOT,ks156/RIOT,alex1818/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,MarkXYang/RIOT,ximus/RIOT,watr-li/RIOT,gebart/RIOT,OTAkeys/RIOT,DipSwitch/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,tdautc19841202/RIOT,l3nko/RIOT,jfischer-phytec-iot/RIOT,Yonezawa-T2/RIOT,aeneby/RIOT,JensErdmann/RIOT,brettswann/RIOT,kaleb-himes/RIOT,sumanpanchal/RIOT,TobiasFredersdorf/RIOT,plushvoxel/RIOT,basilfx/RIOT,tdautc19841202/RIOT,mfrey/RIOT,lebrush/RIOT,chris-wood/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,asanka-code/RIOT,malosek/RIOT,tdautc19841202/RIOT,marcosalm/RIOT,kerneltask/RIOT,ntrtrung/RIOT,cladmi/RIOT,luciotorre/RIOT,neiljay/RIOT,JensErdmann/RIOT,LudwigOrtmann/RIOT,latsku/RIOT,katezilla/RIOT,openkosmosorg/RIOT,koenning/RIOT,bartfaizoltan/RIOT,OTAkeys/RIOT,wentaoshang/RIOT,jbeyerstedt/RIOT-OTA-update,kerneltask/RIOT,beurdouche/RIOT,marcosalm/RIOT,neumodisch/RIOT,authmillenon/RIOT,d00616/RIOT,gbarnett/RIOT,Darredevil/RIOT,kb2ma/RIOT,kaspar030/RIOT,attdona/RIOT,A-Paul/RIOT,watr-li/RIOT,RBartz/RIOT,toonst/RIOT,rfuentess/RIOT,binarylemon/RIOT,benoit-canet/RIOT,gbarnett/RIOT,kbumsik/RIOT,stevenj/RIOT,ntrtrung/RIOT,binarylemon/RIOT,marcosalm/RIOT,x3ro/RIOT,jasonatran/RIOT,FrancescoErmini/RIOT,wentaoshang/RIOT,authmillenon/RIOT,Josar/RIOT,malosek/RIOT,openkosmosorg/RIOT,patkan/RIOT,adrianghc/RIOT,Hyungsin/RIOT-OS,roberthartung/RIOT,ntrtrung/RIOT,jferreir/RIOT,rajma996/RIOT,binarylemon/RIOT,patkan/RIOT,lebrush/RIOT,lazytech-org/RIOT,RubikonAlpha/RIOT,MarkXYang/RIOT,stevenj/RIOT,x3ro/RIOT,josephnoir/RIOT,FrancescoErmini/RIOT,DipSwitch/RIOT,jfischer-phytec-iot/RIOT,bartfaizoltan/RIOT,d00616/RIOT,marcosalm/RIOT,mziegert/RIOT,smlng/RIOT,josephnoir/RIOT,avmelnikoff/RIOT,abkam07/RIOT,kaleb-himes/RIOT,bartfaizoltan/RIOT,mtausig/RIOT,EmuxEvans/RIOT,herrfz/RIOT-old,yogo1212/RIOT,thiagohd/RIOT,backenklee/RIOT,luciotorre/RIOT,sgso/RIOT,brettswann/RIOT,koenning/RIOT,attdona/RIOT,sumanpanchal/RIOT,rakendrathapa/RIOT,benoit-canet/RIOT,adjih/RIOT,OlegHahm/RIOT,rfswarm/RIOT,Lexandro92/RIOT-CoAP,MonsterCode8000/RIOT,altairpearl/RIOT,EmuxEvans/RIOT,alignan/RIOT,brettswann/RIOT,centurysys/RIOT,haoyangyu/RIOT,robixnai/RIOT,abp719/RIOT,syin2/RIOT,Osblouf/RIOT,thomaseichinger/RIOT,Lexandro92/RIOT-CoAP,PSHIVANI/Riot-Code,asanka-code/RIOT,mziegert/RIOT,ThanhVic/RIOT,tfar/RIOT,LudwigOrtmann/RIOT,asanka-code/RIOT,ximus/RIOT,roberthartung/RIOT,arvindpdmn/RIOT,fnack/RIOT,RIOT-OS/RIOT,ant9000/RIOT,ThanhVic/RIOT,gbarnett/RIOT,zhuoshuguo/RIOT,mfrey/RIOT,EmuxEvans/RIOT,bartfaizoltan/RIOT,Hyungsin/RIOT-OS,automote/RIOT,dkm/RIOT,rajma996/RIOT,Ell-i/RIOT,PSHIVANI/Riot-Code,kb2ma/RIOT,robixnai/RIOT,FrancescoErmini/RIOT,rousselk/RIOT,RBartz/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,fnack/RIOT,kbumsik/RIOT,ThanhVic/RIOT,arvindpdmn/RIOT,neumodisch/RIOT,shady33/RIOT,toonst/RIOT,ximus/RIOT,abp719/RIOT,phiros/RIOT,Lexandro92/RIOT-CoAP,zhuoshuguo/RIOT,neiljay/RIOT,attdona/RIOT,alex1818/RIOT,x3ro/RIOT,rousselk/RIOT,MohmadAyman/RIOT,Darredevil/RIOT,plushvoxel/RIOT,thiagohd/RIOT,RBartz/RIOT,latsku/RIOT,JensErdmann/RIOT,cladmi/RIOT,neiljay/RIOT,cladmi/RIOT,l3nko/RIOT,backenklee/RIOT,OlegHahm/RIOT,asanka-code/RIOT,gbarnett/RIOT,x3ro/RIOT,marcosalm/RIOT,gautric/RIOT,hamilton-mote/RIOT-OS,dailab/RIOT,avmelnikoff/RIOT,rousselk/RIOT,lazytech-org/RIOT,immesys/RiSyn,foss-for-synopsys-dwc-arc-processors/RIOT,miri64/RIOT,alignan/RIOT,kaspar030/RIOT,rakendrathapa/RIOT,chris-wood/RIOT,mziegert/RIOT,BytesGalore/RIOT,kushalsingh007/RIOT,EmuxEvans/RIOT,thiagohd/RIOT,benoit-canet/RIOT,thomaseichinger/RIOT,A-Paul/RIOT,ks156/RIOT,watr-li/RIOT,Ell-i/RIOT,gautric/RIOT,kbumsik/RIOT,msolters/RIOT,kYc0o/RIOT,MonsterCode8000/RIOT,arvindpdmn/RIOT,rakendrathapa/RIOT,RubikonAlpha/RIOT,wentaoshang/RIOT,shady33/RIOT,Josar/RIOT,JensErdmann/RIOT,cladmi/RIOT,mtausig/RIOT,adjih/RIOT,Hyungsin/RIOT-OS,biboc/RIOT,gautric/RIOT,rfuentess/RIOT,rakendrathapa/RIOT,benoit-canet/RIOT,adrianghc/RIOT,altairpearl/RIOT,neumodisch/RIOT,sgso/RIOT,kYc0o/RIOT,DipSwitch/RIOT,TobiasFredersdorf/RIOT,brettswann/RIOT,MarkXYang/RIOT,Lexandro92/RIOT-CoAP,DipSwitch/RIOT,thomaseichinger/RIOT,watr-li/RIOT,OlegHahm/RIOT,attdona/RIOT,FrancescoErmini/RIOT,khhhh/RIOT,LudwigOrtmann/RIOT,plushvoxel/RIOT,ximus/RIOT,A-Paul/RIOT,khhhh/RIOT,Darredevil/RIOT,kushalsingh007/RIOT,basilfx/RIOT,ntrtrung/RIOT,changbiao/RIOT,ks156/RIOT,MohmadAyman/RIOT,dailab/RIOT,stevenj/RIOT,Josar/RIOT,hamilton-mote/RIOT-OS,ThanhVic/RIOT,PSHIVANI/Riot-Code,biboc/RIOT,jremmert-phytec-iot/RIOT,jferreir/RIOT,kaspar030/RIOT,jbeyerstedt/RIOT-OTA-update,JensErdmann/RIOT,sgso/RIOT,biboc/RIOT,jfischer-phytec-iot/RIOT,tfar/RIOT,daniel-k/RIOT,thomaseichinger/RIOT,brettswann/RIOT,mtausig/RIOT,RIOT-OS/RIOT,changbiao/RIOT,avmelnikoff/RIOT,RubikonAlpha/RIOT,shady33/RIOT,daniel-k/RIOT,MohmadAyman/RIOT,jasonatran/RIOT,centurysys/RIOT,asanka-code/RIOT,OlegHahm/RIOT,zhuoshuguo/RIOT,miri64/RIOT,LudwigKnuepfer/RIOT,jremmert-phytec-iot/RIOT,alex1818/RIOT,arvindpdmn/RIOT,authmillenon/RIOT,kerneltask/RIOT,thiagohd/RIOT,yogo1212/RIOT,fnack/RIOT,altairpearl/RIOT,MarkXYang/RIOT,Darredevil/RIOT,gbarnett/RIOT,Lexandro92/RIOT-CoAP,Osblouf/RIOT,toonst/RIOT,khhhh/RIOT,altairpearl/RIOT,openkosmosorg/RIOT,herrfz/RIOT,binarylemon/RIOT,koenning/RIOT,FrancescoErmini/RIOT,mziegert/RIOT,DipSwitch/RIOT,toonst/RIOT,aeneby/RIOT,lebrush/RIOT,immesys/RiSyn,dhruvvyas90/RIOT,openkosmosorg/RIOT,herrfz/RIOT-old,koenning/RIOT,phiros/RIOT,jbeyerstedt/RIOT-OTA-update,ks156/RIOT,ant9000/RIOT,MonsterCode8000/RIOT,mfrey/RIOT,malosek/RIOT,yogo1212/RIOT,immesys/RiSyn,neumodisch/RIOT,katezilla/RIOT,stevenj/RIOT,luciotorre/RIOT,dhruvvyas90/RIOT,beurdouche/RIOT,mtausig/RIOT,authmillenon/RIOT,syin2/RIOT,luciotorre/RIOT,jremmert-phytec-iot/RIOT,BytesGalore/RIOT,Ell-i/RIOT,neiljay/RIOT,BytesGalore/RIOT,abp719/RIOT,ks156/RIOT,A-Paul/RIOT,openkosmosorg/RIOT,chris-wood/RIOT,Osblouf/RIOT,jasonatran/RIOT,chris-wood/RIOT,koenning/RIOT,Osblouf/RIOT,fnack/RIOT,ximus/RIOT,jremmert-phytec-iot/RIOT,Yonezawa-T2/RIOT,sumanpanchal/RIOT,patkan/RIOT,syin2/RIOT,immesys/RiSyn,mziegert/RIOT,neiljay/RIOT,rfswarm/RIOT,Osblouf/RIOT,adrianghc/RIOT,abp719/RIOT,phiros/RIOT,LudwigOrtmann/RIOT,adjih/RIOT,benoit-canet/RIOT,authmillenon/RIOT,sgso/RIOT,robixnai/RIOT,wentaoshang/RIOT,herrfz/RIOT-old,OlegHahm/RIOT,rajma996/RIOT,OTAkeys/RIOT,haoyangyu/RIOT,jasonatran/RIOT,thiagohd/RIOT,gebart/RIOT,bartfaizoltan/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,malosek/RIOT,Ell-i/RIOT,MonsterCode8000/RIOT,jferreir/RIOT,fnack/RIOT,EmuxEvans/RIOT,rajma996/RIOT,roberthartung/RIOT,tdautc19841202/RIOT,shady33/RIOT,smlng/RIOT,kb2ma/RIOT,RubikonAlpha/RIOT,herrfz/RIOT,attdona/RIOT,fnack/RIOT,Yonezawa-T2/RIOT,ThanhVic/RIOT,kushalsingh007/RIOT,roberthartung/RIOT,d00616/RIOT,kbumsik/RIOT,changbiao/RIOT,automote/RIOT,herrfz/RIOT,benoit-canet/RIOT,changbiao/RIOT,automote/RIOT,centurysys/RIOT,tdautc19841202/RIOT,Osblouf/RIOT,l3nko/RIOT,RubikonAlpha/RIOT,kerneltask/RIOT,centurysys/RIOT,basilfx/RIOT,alex1818/RIOT,dhruvvyas90/RIOT,rousselk/RIOT,adjih/RIOT,robixnai/RIOT,d00616/RIOT,LudwigKnuepfer/RIOT,kaspar030/RIOT,thomaseichinger/RIOT,stevenj/RIOT,TobiasFredersdorf/RIOT,LudwigKnuepfer/RIOT,jfischer-phytec-iot/RIOT,herrfz/RIOT,ant9000/RIOT,Yonezawa-T2/RIOT,immesys/RiSyn,khhhh/RIOT,herrfz/RIOT-old,RubikonAlpha/RIOT,Darredevil/RIOT,daniel-k/RIOT,khhhh/RIOT,rfuentess/RIOT,MonsterCode8000/RIOT,PSHIVANI/Riot-Code,dkm/RIOT,d00616/RIOT,dkm/RIOT,MonsterCode8000/RIOT,herrfz/RIOT,roberthartung/RIOT,MarkXYang/RIOT,josephnoir/RIOT,miri64/RIOT,chris-wood/RIOT,latsku/RIOT,beurdouche/RIOT,zhuoshuguo/RIOT,changbiao/RIOT,jbeyerstedt/RIOT-OTA-update,ximus/RIOT,kbumsik/RIOT,wentaoshang/RIOT,msolters/RIOT,patkan/RIOT,backenklee/RIOT,watr-li/RIOT,attdona/RIOT,binarylemon/RIOT,jasonatran/RIOT,basilfx/RIOT,sumanpanchal/RIOT,arvindpdmn/RIOT,latsku/RIOT,rfuentess/RIOT,marcosalm/RIOT,zhuoshuguo/RIOT,MohmadAyman/RIOT,toonst/RIOT,yogo1212/RIOT,jferreir/RIOT,aeneby/RIOT,altairpearl/RIOT,avmelnikoff/RIOT,gautric/RIOT,shady33/RIOT,adrianghc/RIOT,Ell-i/RIOT,abkam07/RIOT,latsku/RIOT,RBartz/RIOT,syin2/RIOT,phiros/RIOT,alignan/RIOT,Yonezawa-T2/RIOT,centurysys/RIOT,msolters/RIOT,avmelnikoff/RIOT,cladmi/RIOT,openkosmosorg/RIOT,syin2/RIOT,kaspar030/RIOT,ant9000/RIOT,BytesGalore/RIOT,zhuoshuguo/RIOT,dhruvvyas90/RIOT,rousselk/RIOT,adjih/RIOT,automote/RIOT,Hyungsin/RIOT-OS,mfrey/RIOT,rajma996/RIOT,msolters/RIOT,watr-li/RIOT,Josar/RIOT,automote/RIOT,daniel-k/RIOT,d00616/RIOT,sumanpanchal/RIOT,Hyungsin/RIOT-OS,dkm/RIOT,plushvoxel/RIOT,ntrtrung/RIOT,jferreir/RIOT,katezilla/RIOT,haoyangyu/RIOT,beurdouche/RIOT,abp719/RIOT,sumanpanchal/RIOT,kb2ma/RIOT,luciotorre/RIOT,RBartz/RIOT,rfswarm/RIOT,adrianghc/RIOT,abkam07/RIOT,ntrtrung/RIOT,abkam07/RIOT,brettswann/RIOT,smlng/RIOT,changbiao/RIOT,A-Paul/RIOT,MohmadAyman/RIOT,wentaoshang/RIOT,kaleb-himes/RIOT,LudwigOrtmann/RIOT,hamilton-mote/RIOT-OS,mtausig/RIOT,TobiasFredersdorf/RIOT,kaleb-himes/RIOT,rakendrathapa/RIOT,dkm/RIOT,immesys/RiSyn,MohmadAyman/RIOT,hamilton-mote/RIOT-OS,patkan/RIOT,malosek/RIOT,stevenj/RIOT,kYc0o/RIOT,neumodisch/RIOT,malosek/RIOT,dhruvvyas90/RIOT,BytesGalore/RIOT,msolters/RIOT,josephnoir/RIOT,LudwigKnuepfer/RIOT