pgsql: Add note that LIMIT without ORDER BY can produce outright

Log Message:
-----------
Add note that LIMIT without ORDER BY can produce outright nondeterministic
results. Necessary due to introduction of syncscan patch.

Modified Files:
--------------
pgsql/doc/src/sgml/ref:
select.sgml (r1.100 -> r1.101)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/select.sgml.diff?r1=1.100&r2=1.101)