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

Log Message:
-----------
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.

Modified Files:
--------------
pgsql/doc/src/sgml:
config.sgml (r1.304 -> r1.305)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.304&r2=1.305)
pgsql/src/backend/storage/ipc:
standby.c (r1.28 -> r1.29)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/standby.c?r1=1.28&r2=1.29)
pgsql/src/backend/utils/error:
elog.c (r1.225 -> r1.226)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/error/elog.c?r1=1.225&r2=1.226)
pgsql/src/backend/utils/misc:
guc.c (r1.567 -> r1.568)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.567&r2=1.568)