Log Message:
-----------
Clarify STABLE function documentation to highlight how such functions
can be optimized.
Modified Files:
--------------
pgsql/doc/src/sgml:
xfunc.sgml (r1.109 -> r1.110)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/xfunc.sgml.diff?r1=1.109&r2=1.110)