pgsql: Document that forking while having open libpq connections is not

Log Message:
-----------
Document that forking while having open libpq connections is not
advised.

Modified Files:
--------------
pgsql/doc/src/sgml:
libpq.sgml (r1.288 -> r1.289)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/libpq.sgml?r1=1.288&r2=1.289)