pgsql: Bring some sanity to the trace_recovery_messages code and docs.

Log Message:
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Bring some sanity to the trace_recovery_messages code and docs.
Per gripe from Fujii Masao, though this is not exactly his proposed patch.
Categorize as DEVELOPER_OPTIONS and set context PGC_SIGHUP, as per Fujii,
but set the default to LOG because higher values aren't really sensible
(see the code for trace_recovery()). Fix the documentation to agree with
the code and to try to explain what the variable actually does. Get rid
of no-op calls trace_recovery(LOG), which accomplish nothing except to
demonstrate that this option confuses even its author.

Tags:
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REL9_0_STABLE

Modified Files:
--------------
pgsql/doc/src/sgml:
config.sgml (r1.294.2.4 -> r1.294.2.5)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.294.2.4&r2=1.294.2.5)
pgsql/src/backend/storage/ipc:
standby.c (r1.27.2.1 -> r1.27.2.2)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/standby.c?r1=1.27.2.1&r2=1.27.2.2)
pgsql/src/backend/utils/error:
elog.c (r1.224.2.1 -> r1.224.2.2)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/error/elog.c?r1=1.224.2.1&r2=1.224.2.2)
pgsql/src/backend/utils/misc:
guc.c (r1.561 -> r1.561.2.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.561&r2=1.561.2.1)