pgsql: Document that after triggers that need to see changed rows should

Log Message:
-----------
Document that after triggers that need to see changed rows should not be
marked stable.

Modified Files:
--------------
pgsql/doc/src/sgml/ref:
create_function.sgml (r1.90 -> r1.91)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/create_function.sgml?r1=1.90&r2=1.91)