pgsql: Rewrite discussion of ORDER BY to emphasize the SQL99 expression

Log Message:
-----------
Rewrite discussion of ORDER BY to emphasize the SQL99 expression case
instead of the SQL92 output-column-ID case.

Modified Files:
--------------
pgsql/doc/src/sgml:
queries.sgml (r1.38 -> r1.39)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/queries.sgml.diff?r1=1.38&r2=1.39)