pgsql: Move table constraint patch up into the main new features section

Log Message:
-----------
Move table constraint patch up into the main new features section of the
release notes.

Modified Files:
--------------
pgsql/doc/src/sgml:
release.sgml (r1.370 -> r1.371)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release.sgml.diff?r1=1.370&r2=1.371)