etcdserver: document DeleteRangeRequest prefixes
etcdserver: document DeleteRangeRequest prefixes

There was missing info about deleting prefixes in the proto docs for
DeleteRangeRequest.

Closes #6641.

cockroachdb/etcd,akauppi/etcd,ravigadde/etcd,lorneli/etcd,qqsun8819/etcd,ravigadde/etcd,sebrandon1/etcd,lumjjb/etcd,xiang90/etcd,dannysauer/etcd,joshix/etcd,insequent/etcd,mqliang/etcd,cockroachdb/etcd,bdarnell/etcd,bdarnell/etcd,hongchaodeng/etcd,endocode/etcd,sinsharat/etcd,fanminshi/etcd,sebrandon1/etcd,handsomesun/etcd,glevand/coreos--etcd,mqliang/etcd,visheshnp/etcd,YuleiXiao/etcd,gyuho/etcd,dannysauer/etcd,OyTao/etcd_learning,etcd-io/etcd,CrazyJvm/etcd,lorneli/etcd,nekto0n/etcd,whuwxl/etcd,etcd-io/etcd,li-ang/etcd,li-ang/etcd,qqsun8819/etcd,YuleiXiao/etcd,gyuho/etcd,heyitsanthony/etcd,coreos/etcd,heyitsanthony/etcd,rchicoli/etcd,vimalk78/etcd,nekto0n/etcd,rnaveiras/etcd,bluebreezecf/etcd,glevand/coreos--etcd,handsomesun/etcd,armstrongli/etcd,akauppi/etcd,rnaveiras/etcd,insequent/etcd,a-robinson/etcd,mickep76/etcd,visheshnp/etcd,coreos/etcd,lumjjb/etcd,ajityagaty/etcd,armstrongli/etcd,tguangch/etcd,ajityagaty/etcd,rchicoli/etcd,yudai/etcd,a-robinson/etcd,mickep76/etcd,sinsharat/etcd,OyTao/etcd_learning,etcd-io/etcd,plasticbox/etcd,mqliang/etcd,jlamillan/etcd,xiang90/etcd,mkumatag/etcd,CrazyJvm/etcd,tguangch/etcd,whuwxl/etcd,yudai/etcd,joshix/etcd,hongchaodeng/etcd,rnaveiras/etcd,endocode/etcd,mkumatag/etcd,jlamillan/etcd,mkumatag/etcd,vimalk78/etcd,plasticbox/etcd,bluebreezecf/etcd,ajityagaty/etcd,fanminshi/etcd