Document Raft.send method.
Document Raft.send method.

The change makes it explicit that sending messages does not happen
immidietely and is subject to proper persist & then send protocol
on the application side. See:

https://github.com/etcd-io/etcd/issues/12589#issuecomment-752867024

for more context.

etcd-io/etcd,mkumatag/etcd,rnaveiras/etcd,mkumatag/etcd,mqliang/etcd,gyuho/etcd,ajityagaty/etcd,mkumatag/etcd,gyuho/etcd,coreos/etcd,etcd-io/etcd,etcd-io/etcd,ajityagaty/etcd,rnaveiras/etcd,rnaveiras/etcd,mqliang/etcd,coreos/etcd,ajityagaty/etcd,mqliang/etcd