Documentation/dev-guide: highlight gRPC gateway endpoint changes
Documentation/dev-guide: highlight gRPC gateway endpoint changes

Signed-off-by: Gyuho Lee <3454051ee3341eb69bdf6fca40899cdad4d86496@gmail.com>

etcd-io/etcd,lorneli/etcd,sinsharat/etcd,coreos/etcd,xiang90/etcd,mkumatag/etcd,lorneli/etcd,vimalk78/etcd,gyuho/etcd,ajityagaty/etcd,glevand/coreos--etcd,ajityagaty/etcd,cockroachdb/etcd,xiang90/etcd,glevand/coreos--etcd,fanminshi/etcd,rnaveiras/etcd,li-ang/etcd,etcd-io/etcd,sebrandon1/etcd,dannysauer/etcd,mkumatag/etcd,etcd-io/etcd,mkumatag/etcd,sebrandon1/etcd,mqliang/etcd,rnaveiras/etcd,bdarnell/etcd,sinsharat/etcd,dannysauer/etcd,rnaveiras/etcd,mqliang/etcd,gyuho/etcd,joshix/etcd,ajityagaty/etcd,mqliang/etcd,joshix/etcd,coreos/etcd,bdarnell/etcd,cockroachdb/etcd,li-ang/etcd,fanminshi/etcd,vimalk78/etcd