Update MERGE docs to mention that ONLY is supported.

Update MERGE docs to mention that ONLY is supported.

Commit 7103ebb7aa added support for MERGE, which included support for
inheritance hierarchies, but didn't document the fact that ONLY could
be specified before the source and/or target tables to exclude tables
inheriting from the tables specified.

Update merge.sgml to mention this, and while at it, add some
regression tests to cover it.

Dean Rasheed, reviewed by Nathan Bossart.

Backpatch to 15, where MERGE was added.

Discussion: https://postgr.es/m/CAEZATCU0XM-bJCvpJuVRU3UYNRqEBS6g4-zH%3Dj9Ye0caX8F6uQ%40mail.gmail.com