pgsql-server: log_destination should only be PGC_SIGHUP, not

Log Message:
-----------
log_destination should only be PGC_SIGHUP, not PGC_POSTMASTER, as per
discussion yesterday. Also a few improvements in the associated
documentation.

Modified Files:
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pgsql-server/doc/src/sgml:
maintenance.sgml (r1.35 -> r1.36)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/doc/src/sgml/maintenance.sgml.diff?r1=1.35&r2=1.36)
runtime.sgml (r1.269 -> r1.270)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/doc/src/sgml/runtime.sgml.diff?r1=1.269&r2=1.270)
pgsql-server/src/backend/utils/misc:
guc.c (r1.223 -> r1.224)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/utils/misc/guc.c.diff?r1=1.223&r2=1.224)