add rollback explanation to docker service update command document
add rollback explanation to docker service update command document

Signed-off-by: erxian <705d14bb9cd952b3e3d087bfc56ee0436b075225@gmail.com>

lixiaobing10051267/docker,estesp/docker,vdemeester/docker,HideoYamauchi/docker,trebonian/docker,maxamillion/docker,gnailuy/docker,YuPengZTE/docker,jlhawn/docker,nawawi/docker,kencochrane/docker,xuxinkun/docker,PatrickLang/moby,akira-baruah/docker,yongtang/docker,konstruktoid/docker-1,djs55/docker,unclejack/docker,sdurrheimer/docker,pramodhkp/moby,resin-io/docker,allencloud/docker,gjaskiewicz/moby,PatrickLang/moby,thtanaka/docker,liaoqingwei/docker,tklauser/docker,kojiromike/docker,coreos/docker,amatsus/docker,haveatry/docker,tonistiigi/docker,AkihiroSuda/docker,kojiromike/docker,djs55/docker,gnailuy/docker,jimjag/docker,shhsu/docker,kencochrane/docker,tophj-ibm/moby,sdurrheimer/docker,coolljt0725/docker,ibuildthecloud/docker,gjaskiewicz/moby,yosifkit/docker,nalind/docker,nalind/docker,noironetworks/docker,ibuildthecloud/docker,KingEmet/docker,dccurtis/docker,akira-baruah/docker,jlhawn/docker,lixiaobing10051267/docker,diogomonica/docker,Mokolea/docker,shishir-a412ed/docker,jameszhan/docker,Ramzec/docker,ripcurld0/moby,yummypeng/docker,duglin/docker,konstruktoid/docker-upstream,unclejack/docker,ibuildthecloud/docker,x1022as/docker,maxamillion/docker,kencochrane/docker,mixja/docker,rhatdan/docker,ColinHebert/docker,jwhonce/docker,resin-io/docker,cpuguy83/docker,jvanz/docker,laijs/moby,kolyshkin/moby,gnailuy/docker,yummypeng/docker,xuxinkun/docker,kolyshkin/docker,alfred-landrum/docker,sterchelen/docker,cyphar/docker,dims/docker,cpuguy83/docker,gschaetz/moby,tklauser/docker,trebonian/docker,mixja/docker,tophj-ibm/docker,haveatry/docker,PatrickLang/moby,crosbymichael/docker,scotttlin/docker,yosifkit/docker,dperny/docker,dgageot/docker,mahak/docker,jimjag/docker,mike1729/docker,pradipd/moby,bonczj/moby-json-file-logger,docker/docker,jvanz/docker,mike1729/docker,tonistiigi/docker,Ramzec/docker,maxamillion/docker,q384566678/docker,rancher/docker,gjaskiewicz/moby,ripcurld0/moby,dims/docker,jameszhan/docker,rancher/docker,tianon/docker,resin-io/docker,srust/docker,nawawi/docker,Mokolea/docker,miminar/docker,imkin/moby,rremer/moby,dgageot/docker,catinthesky/docker,lixiaobing1/docker,nawawi/docker,kojiromike/docker,danyalKhaliq10p/moby,jimjag/docker,kolyshkin/docker,raja-sami-10p/moby,jwhonce/docker,bonczj/moby-json-file-logger,vdemeester/docker,qudongfang/docker,justincormack/docker,nalind/docker,q384566678/docker,vdemeester/docker,sterchelen/docker,flavio/docker,mkumatag/docker,moby/moby,konstruktoid/docker-1,liaoqingwei/docker,michael-holzheu/docker,jameszhan/docker,shhsu/docker,raja-sami-10p/moby,djs55/docker,qudongfang/docker,thtanaka/docker,amatsus/docker,ndeloof/docker,whuwxl/docker,ibuildthecloud/docker,erikstmartin/docker,POLJM242/docker,rremer/moby,Ramzec/docker,moypray/docker,JMesser81/docker,dmcgowan/docker,michael-holzheu/docker,gschaetz/moby,bearstech/moby,whuwxl/docker,HideoYamauchi/docker,imkin/moby,YuPengZTE/docker,coreos/docker,ffoysal/docker,Syntaxide/docker,diogomonica/docker,trebonian/docker,tmp6154/docker,cpuguy83/docker,johnstep/moby-moby,KingEmet/docker,hqhq/moby,duglin/docker,SvenDowideit/docker,SvenDowideit/docker,gnailuy/docker,ColinHebert/docker,nalind/docker,moul/docker,madhanrm/docker,cyphar/docker,fcrisciani/docker,erikh/docker,darrenstahlmsft/docker,mkumatag/docker,HideoYamauchi/docker,naveed-jamil-tenpearls/moby,tophj-ibm/docker,whuwxl/docker,bearstech/moby,darrenstahlmsft/moby,moypray/docker,x1022as/docker,prertik/docker,thtanaka/docker,qudongfang/docker,moul/docker,AkihiroSuda/docker,pradipd/moby,dperny/docker,jvanz/docker,konstruktoid/docker-upstream,dgageot/docker,srust/docker,scotttlin/docker,dccurtis/docker,allencloud/docker,raja-sami-10p/moby,flavio/docker,tklauser/docker,mstanleyjones/docker,ffoysal/docker,tonistiigi/docker,unclejack/docker,bonczj/moby-json-file-logger,kolyshkin/moby,konstruktoid/docker-upstream,sarnautov/moby,thaJeztah/docker,jimjag/docker,kolyshkin/moby,dmcgowan/docker,madhanrm/docker,JMesser81/docker,dccurtis/docker,srust/docker,darrenstahlmsft/moby,konstruktoid/docker-1,charleszheng44/moby,KarthikNayak/docker,estesp/docker,justincormack/docker,ndeloof/docker,thaJeztah/docker,imkin/moby,justincormack/docker,yasiramin10p/moby,paralin/docker,laijs/moby,konstruktoid/docker-upstream,flavio/docker,moypray/docker,AkihiroSuda/docker,tophj-ibm/docker,Mokolea/docker,Syntaxide/docker,yasiramin10p/moby,hqhq/moby,Ramzec/docker,JMesser81/docker,coolljt0725/docker,lixiaobing1/docker,cyphar/docker,crosbymichael/docker,q384566678/docker,cpuguy83/docker,laijs/docker,moby/moby,scotttlin/docker,bearstech/moby,tophj-ibm/moby,yosifkit/docker,ndeloof/docker,sarnautov/moby,Syntaxide/docker,rancher/docker,docker/docker,shhsu/docker,fcrisciani/docker,erikh/docker,duglin/docker,dperny/docker,paralin/docker,noironetworks/docker,catinthesky/docker,crosbymichael/docker,sterchelen/docker,thaJeztah/docker,SvenDowideit/docker,erikh/docker,jthelin/docker,mikebrow/docker,x1022as/docker,dmcgowan/docker,dims/docker,pramodhkp/moby,dsheets/moby,paralin/docker,shishir-a412ed/docker,amatsus/docker,YuPengZTE/docker,moby/moby,yasiramin10p/moby,mixja/docker,erikstmartin/docker,shishir-a412ed/docker,yosifkit/docker,coolljt0725/docker,alfred-landrum/docker,pramodhkp/moby,naveed-jamil-tenpearls/moby,coreos/docker,moby/moby,tophj-ibm/moby,dsheets/moby,ColinHebert/docker,tmp6154/docker,haveatry/docker,jwhonce/docker,darrenstahlmsft/docker,danyalKhaliq10p/moby,tmp6154/docker,mkumatag/docker,dsheets/moby,gschaetz/moby,michael-holzheu/docker,johnstep/moby-moby,diogomonica/docker,paralin/docker,prertik/docker,KingEmet/docker,yongtang/docker,thaJeztah/docker,fcrisciani/docker,noironetworks/docker,sarnautov/moby,mahak/docker,mstanleyjones/docker,darrenstahlmsft/docker,johnstep/moby-moby,mahak/docker,ffoysal/docker,danyalKhaliq10p/moby,docker/docker,mstanleyjones/docker,KarthikNayak/docker,bearstech/moby,mtesselH/docker,jlhawn/docker,charleszheng44/moby,rremer/moby,estesp/docker,POLJM242/docker,ibuildthecloud/docker,erikstmartin/docker,KarthikNayak/docker,yummypeng/docker,sdurrheimer/docker,lixiaobing10051267/docker,nawawi/docker,mike1729/docker,liaoqingwei/docker,laijs/docker,rhatdan/docker,mtesselH/docker,ripcurld0/moby,moul/docker,mikebrow/docker,prertik/docker,moul/docker,xuxinkun/docker,AkihiroSuda/docker,Mokolea/docker,allencloud/docker,catinthesky/docker,rhatdan/docker,mikebrow/docker,madhanrm/docker,yongtang/docker,miminar/docker,alfred-landrum/docker,mahak/docker,flavio/docker,tianon/docker,naveed-jamil-tenpearls/moby,miminar/docker,laijs/docker,darrenstahlmsft/moby,pradipd/moby,mtesselH/docker,charleszheng44/moby,jthelin/docker,hqhq/moby,trebonian/docker,laijs/moby,lixiaobing1/docker