doc: List all the possible values of pg_stat_wal_receiver.status

doc: List all the possible values of pg_stat_wal_receiver.status

The possible values of pg_stat_wal_receiver.status have never been
documented.  Note that the status "stopped" will never show up in this
view, hence there is no need to document it.

Issue noticed while discussing a patch that aims to add a new status to
WAL receiver.

Author: Xuneng Zhou <xunengzhou@gmail.com>
Discussion: https://postgr.es/m/CABPTF7X_Jgmyk1FBVNf3tyAOKqU55LLpLMzWkGtEAb_jQWVN=g@mail.gmail.com