raft: document why reuse candidate's term for vote response in stepCandidate
raft: document why reuse candidate's term for vote response in stepCandidate

"stepCandidate" should reuse candidate's own term, not term in Message,
because pre-vote is requested with future term.

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

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