pgsql: Add a note that regression test row ordering differences can be

Log Message:
-----------
Add a note that regression test row ordering differences can be caused
by non-default planner parameter settings.

Modified Files:
--------------
pgsql/doc/src/sgml:
regress.sgml (r1.45 -> r1.46)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/regress.sgml.diff?r1=1.45&r2=1.46)