pgsql: Doc: Miscellaneous doc updates for MERGE.

Doc: Miscellaneous doc updates for MERGE.

Update a few places in the documentation that should mention MERGE
among the list of applicable commands. In a couple of places, a
slightly more detailed description of what happens for MERGE seems
appropriate.

Reviewed by Alvaro Herrera.

Discussion: http://postgr.es/m/CAEZATCWqHLcxab89ATMQZNGFG_mxDPM%2BjzkSbXKD3JYPfRGvtw%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ee7e8f3838733f09a67e1421f0d7c6498cd68ab0

Modified Files
--------------
doc/src/sgml/arch-dev.sgml               | 16 ++++++++++++----
doc/src/sgml/ddl.sgml                    |  3 ++-
doc/src/sgml/high-availability.sgml      |  3 ++-
doc/src/sgml/perform.sgml                |  8 +++++---
doc/src/sgml/plpgsql.sgml                |  3 ++-
doc/src/sgml/protocol.sgml               |  7 +++++++
doc/src/sgml/queries.sgml                | 12 +++++++-----
doc/src/sgml/ref/create_publication.sgml |  6 ++++++
doc/src/sgml/ref/explain.sgml            |  6 ++++--
doc/src/sgml/ref/prepare.sgml            |  3 ++-
doc/src/sgml/ref/set_transaction.sgml    | 11 +++++++----
doc/src/sgml/xfunc.sgml                  |  3 ++-
12 files changed, 58 insertions(+), 23 deletions(-)