doc: Clean up some recently added PL/pgSQL documentation

- Capitalize titles consistently.
- Fix some grammar.
- Group "Obtaining Information About an Error" under "Trapping Errors",
  but make "Obtaining the Call Stack Context Information" its own
  section, since it's not about errors.
Fix mishandling of background worker PGPROCs in EXEC_BACKEND builds.

InitProcess() relies on IsBackgroundWorker to decide whether the PGPROC
for a new backend should be taken from ProcGlobal's freeProcs or from
bgworkerFreeProcs.  In EXEC_BACKEND builds, InitProcess() is called
sooner than in non-EXEC_BACKEND builds, and IsBackgroundWorker wasn't
getting initialized soon enough.

Report by Noah Misch.  Diagnosis and fix by me.
Avoid wholesale autovacuuming when autovacuum is nominally off.

When autovacuum is nominally off, we will still launch autovac workers
to vacuum tables that are at risk of XID wraparound.  But after we'd done
that, an autovac worker would proceed to autovacuum every table in the
targeted database, if they meet the usual thresholds for autovacuuming.
This is at best pretty unexpected; at worst it delays response to the
wraparound threat.  Fix it so that if autovacuum is nominally off, we
*only* do forced vacuums and not any other work.

Per gripe from Andrey Zhidenkov.  This has been like this all along,
so back-patch to all supported branches.
Fix typo in user manual
Fix bug in pg_receivexlog --verbose.

In 9.2, pg_receivexlog with verbose option has emitted the messages
at the end of each WAL file. But the commit 0b63291 suppressed such
messages by mistake. This commit fixes the bug so that pg_receivexlog
--verbose outputs such messages again.

Back-patch to 9.3 where the bug was added.
Add missing PQclear() calls into pg_receivexlog.

Back-patch to 9.3.
pg_upgrade:  remove reference to autovacuum_multixact_freeze_max_age

autovacuum_multixact_freeze_max_age was added as a pg_ctl start
parameter in 9.3.X to prevent autovacuum from running.  However, only
some 9.3.X releases have autovacuum_multixact_freeze_max_age as it was
added in a minor PG 9.3 release.  It also isn't needed because -b turns
off autovacuum in 9.1+.

Without this fix, trying to upgrade from an early 9.3 release to 9.4
would fail.

Report by EDB

Backpatch through 9.3
Improve some JSON error messages.

These messages are new in 9.4, which hasn't been released yet, so
back-patch to REL9_4_STABLE.

Daniele Varrazzo
Change ParseConfigFp() so that it doesn't process unused entry of each parameter.

When more than one setting entries of same parameter exist in the
configuration file, PostgreSQL uses only entry appearing last in
configuration file scan. Since the other entries are not used,
ParseConfigFp() doesn't need to process them, but previously it did
that. This problematic behavior caused the configuration file scan
to detect invalid settings of unused entries (e.g., existence of
multiple entries of PGC_POSTMASTER parameter) and log the messages
complaining about them.

This commit changes the configuration file scan so that it processes
only last entry of each parameter.

Note that when multiple entries of same parameter exist both in
postgresql.conf and postgresql.auto.conf, unused entries in
postgresql.conf are still processed only at postmaster startup.

The problem has existed since old version, but a user is more likely
to encounter it since 9.4 where ALTER SYSTEM command was introduced.
So back-patch to 9.4.

Amit Kapila, slightly modified by me. Per report from Christoph Berg.
Fix alternate regression test output file.

Commit 930e8ad738f5c78abd667626ca86f2553f36356f broke this.

Jeff Janes
pg_upgrade: prevent oid conflicts with new-cluster TOAST tables

Previously, TOAST tables only required in the new cluster could cause
oid conflicts if they were auto-numbered and a later conflicting oid had
to be assigned.

Backpatch through 9.3
Fix typo in docs.

s/XIDs XIDs/XIDs/ in one place in maintenance.sgml.

Guillaume Lelarge