pgsql: Change a couple of ill-advised uses of INFO elog level to

Log Message:
-----------
Change a couple of ill-advised uses of INFO elog level to WARNINGs; in
particular this allows EmitWarningsOnPlaceholders messages to show up in the
postmaster log by default.  Update elog.h comment to make it clearer what INFO
is for, and fix one example in the SGML docs that was misusing it.  Per my
gripe of yesterday.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        trigger.sgml (r1.53 -> r1.54)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/trigger.sgml?r1=1.53&r2=1.54)
    pgsql/src/backend/utils/misc:
        guc.c (r1.489 -> r1.490)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.489&r2=1.490)
    pgsql/src/include/utils:
        elog.h (r1.98 -> r1.99)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/elog.h?r1=1.98&r2=1.99)