gnrc_sixlowpan_frag/rbuf: document undocumented functions
gnrc_sixlowpan_frag/rbuf: document undocumented functions

This adds documentation for the following undocumented functions:

- rbuf_rm()
- rbuf_entry_empty()

rfuentess/RIOT,x3ro/RIOT,RIOT-OS/RIOT,toonst/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,basilfx/RIOT,kYc0o/RIOT,basilfx/RIOT,rfuentess/RIOT,basilfx/RIOT,rfuentess/RIOT,toonst/RIOT,jasonatran/RIOT,OlegHahm/RIOT,smlng/RIOT,smlng/RIOT,smlng/RIOT,authmillenon/RIOT,cladmi/RIOT,miri64/RIOT,mtausig/RIOT,kaspar030/RIOT,yogo1212/RIOT,josephnoir/RIOT,ant9000/RIOT,mfrey/RIOT,aeneby/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,ant9000/RIOT,authmillenon/RIOT,josephnoir/RIOT,mfrey/RIOT,smlng/RIOT,basilfx/RIOT,rfuentess/RIOT,basilfx/RIOT,authmillenon/RIOT,ant9000/RIOT,kaspar030/RIOT,ant9000/RIOT,kaspar030/RIOT,kaspar030/RIOT,josephnoir/RIOT,x3ro/RIOT,miri64/RIOT,mtausig/RIOT,cladmi/RIOT,aeneby/RIOT,RIOT-OS/RIOT,smlng/RIOT,authmillenon/RIOT,josephnoir/RIOT,mtausig/RIOT,yogo1212/RIOT,aeneby/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,aeneby/RIOT,mtausig/RIOT,jasonatran/RIOT,aeneby/RIOT,toonst/RIOT,kYc0o/RIOT,mfrey/RIOT,OlegHahm/RIOT,mfrey/RIOT,yogo1212/RIOT,OTAkeys/RIOT,yogo1212/RIOT,cladmi/RIOT,OlegHahm/RIOT,mfrey/RIOT,miri64/RIOT,yogo1212/RIOT,rfuentess/RIOT,authmillenon/RIOT,jasonatran/RIOT,OTAkeys/RIOT,toonst/RIOT,x3ro/RIOT,ant9000/RIOT,cladmi/RIOT,kaspar030/RIOT,miri64/RIOT,authmillenon/RIOT,OlegHahm/RIOT,miri64/RIOT,toonst/RIOT,kYc0o/RIOT,x3ro/RIOT,OTAkeys/RIOT,x3ro/RIOT,josephnoir/RIOT,OTAkeys/RIOT,mtausig/RIOT,cladmi/RIOT,kYc0o/RIOT,jasonatran/RIOT,OTAkeys/RIOT,jasonatran/RIOT