Further Hot Standby documentation required

Recent changes to parameters aren't yet sufficiently well documented and
I'd like to see this improved by the authors of those patches. I accept
the behaviour changes, but we need full docs to explain them.

* hot_standby doesn't mention that wal_level = hot_standby is also
required, nor is there a xref.

* wal_level = 'hot_standby' doesn't mention that the second parameter
also needs to be set, nor is there a xref.

* wal_level doesn't describe what the impacts are on a standby if the
level is changed on the primary, nor is there a caution or a warning of
any kind. For example, if a standby is setup with hot_standby = on and
the primary is set wal_level = archive, does the standby start working
if the primary changes wal_level = hot_standby? What happens if the
primary is set wal_level = hot_standby and is then changed to archive?

* wal_level doesn't explicitly describe that the levels are in sequence
and that hot_standby is a superset of archive. The comment "so at least
archive level must be used to enable .... streaming replication." can be
misinterpreted to mean that hot_standby level cannot be used with SR,
unless the "at least archive level" is successfully interpreted with the
understanding that minimal < archive < hot_standby. So would like to see
that made explicit.

-- Simon Riggs           www.2ndQuadrant.com