pgsql: Add a note pointing out that is_pseudo_constant_clause() doesn't

Log Message:
-----------
Add a note pointing out that is_pseudo_constant_clause() doesn't check
for aggregates. This is OK for current uses but could burn somebody
someday...

Modified Files:
--------------
pgsql/src/backend/optimizer/util:
clauses.c (r1.229 -> r1.230)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/clauses.c.diff?r1=1.229&r2=1.230)