Doc: warn that timezone abbreviations don't work in recovery_target_time.

Doc: warn that timezone abbreviations don't work in recovery_target_time.

Moving this setting into the main configuration file was ill-considered,
perhaps, because that typically causes it to be set before
timezone_abbreviations has been set.  Which in turn means that zone
abbreviations don't work, only full zone names.

We could imagine hacking things so that such cases do work, but the
stability of the hack would be questionable, and the value isn't really
that high.  Instead just document that you should use a numeric zone
offset or a full zone name.

Per bug #16404 from Reijo Suhonen.
Back-patch to v12 where this was changed.

Discussion: https://postgr.es/m/16404-4603a99603fbd04c@postgresql.org