Adding in docs the meaning of pg_stat_replication.sync_state

Hi all,

The documentation does not explain at all what means "sync" or "async"
on pg_stat_replication. The paragraph "Planning for high availability"
mentions "catchup" and "streaming", but it does not say that users can
refer to it directly in pg_stat_replication.

Thoughts about the patch attached to add a couple of sentences in
"Planning for high availability"? We could as well mention it directly
in the page of pg_stat_replication but this information seems more
suited if located in the HA section.

Thanks,
--
Michael