raft/doc: add notice about thread safety of messages
raft/doc: add notice about thread safety of messages

Fixes #4285
siddontang/etcd,mickep76/etcd,sinsharat/etcd,jlamillan/etcd,a-robinson/etcd,achanda/etcd,mqliang/etcd,etcd-io/etcd,insequent/etcd,purpleidea/etcd,hongchaodeng/etcd,visheshnp/etcd,mqliang/etcd,heyitsanthony/etcd,armstrongli/etcd,tamird/etcd,gyuho/etcd,nekto0n/etcd,YuleiXiao/etcd,OyTao/etcd_learning,swingbach/etcd,yudai/etcd,OyTao/etcd_learning,jonboulle/etcd,etcd-io/etcd,rchicoli/etcd,cockroachdb/etcd,siddontang/etcd,coreos/etcd,linuxcer/etcd,gyuho/etcd,swingbach/etcd,vimalk78/etcd,StorReduce/etcd,coreos/etcd,jonboulle/etcd,StorReduce/etcd,westhood/etcd,YuleiXiao/etcd,lorneli/etcd,heyitsanthony/etcd,li-ang/etcd,tamird/etcd,plasticbox/etcd,li-ang/etcd,whuwxl/etcd,CrazyJvm/etcd,shawnps/etcd,ktateish/etcd,ravigadde/etcd,dnaeon/etcd,handsomesun/etcd,westhood/etcd,jlamillan/etcd,tguangch/etcd,joshix/etcd,linuxcer/etcd,qqsun8819/etcd,ajityagaty/etcd,qqsun8819/etcd,rnaveiras/etcd,fanminshi/etcd,plasticbox/etcd,lumjjb/etcd,dnaeon/etcd,yudai/etcd,whuwxl/etcd,sofuture/etcd,zaphod-concur/etcd,bluebreezecf/etcd,endocode/etcd,wantonsolutions/dinv-etcd,insequent/etcd,mkumatag/etcd,rchicoli/etcd,handsomesun/etcd,glycerine/etcd,zaphod-concur/etcd,bdarnell/etcd,wantonsolutions/dinv-etcd,cockroachdb/etcd,fanminshi/etcd,vimalk78/etcd,lumjjb/etcd,bdarnell/etcd,rnaveiras/etcd,mkumatag/etcd,hongchaodeng/etcd,raoofm/etcd,armstrongli/etcd,joshix/etcd,glycerine/etcd,akauppi/etcd,chipironcin/etcd,mickep76/etcd,luxas/etcd,wantonsolutions/dinv-etcd,etcd-io/etcd,achanda/etcd,ajityagaty/etcd,bluebreezecf/etcd,raoofm/etcd,purpleidea/etcd,visheshnp/etcd,mqliang/etcd,sinsharat/etcd,lorneli/etcd,ravigadde/etcd,dannysauer/etcd,ajityagaty/etcd,xiang90/etcd,chipironcin/etcd,sofuture/etcd,nekto0n/etcd,akauppi/etcd,CrazyJvm/etcd,rnaveiras/etcd,xiang90/etcd,endocode/etcd,shawnps/etcd,dannysauer/etcd,sebrandon1/etcd,tguangch/etcd,mkumatag/etcd,ktateish/etcd,luxas/etcd,glevand/coreos--etcd,a-robinson/etcd,sebrandon1/etcd,glevand/coreos--etcd