doc: Improve a couple of places in the MERGE docs.

doc: Improve a couple of places in the MERGE docs.

In the synopsis, make the syntax for merge_update consistent with the
syntax for a plain UPDATE command. It was missing the optional "ROW"
keyword that can be used in a multi-column assignment, and the option
to assign from a multi-column subquery, both of which have been
supported by MERGE since it was introduced.

In the parameters section for the with_query parameter, mention that
WITH RECURSIVE isn't supported, since this is different from plain
INSERT, UPDATE, and DELETE commands. While at it, move that entry to
the top of the list, for consistency with the other pages.

Back-patch to v15, where MERGE was introduced.

Discussion: https://postgr.es/m/CAEZATCWoQyWkMFfu7JXXQr8dA6%3DgxjhYzgpuBP2oz0QoJTxGWw%40mail.gmail.com