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

doc/src/sgml/ref/create_publication.sgml

@@ -306,6 +306,12 @@ CREATE PUBLICATION <replaceable class="parameter">name</replaceable>
    <command>UPDATE</command>, or it may not be published at all.
   </para>
 
+  <para>
+   For a <command>MERGE</command> command, the publication will publish an
+   <command>INSERT</command>, <command>UPDATE</command>, or <command>DELETE</command>
+   for each row inserted, updated, or deleted.
+  </para>
+
   <para>
    <command>ATTACH</command>ing a table into a partition tree whose root is
    published using a publication with <literal>publish_via_partition_root</literal>

doc/src/sgml/ref/explain.sgml

@@ -94,7 +94,8 @@ EXPLAIN [ ANALYZE ] [ VERBOSE ] <replaceable class="parameter">statement</replac
     statement will happen as usual.  If you wish to use
     <command>EXPLAIN ANALYZE</command> on an
     <command>INSERT</command>, <command>UPDATE</command>,
-    <command>DELETE</command>, <command>CREATE TABLE AS</command>,
+    <command>DELETE</command>, <command>MERGE</command>,
+    <command>CREATE TABLE AS</command>,
     or <command>EXECUTE</command> statement
     without letting the command affect your data, use this approach:
 <programlisting>
@@ -272,7 +273,8 @@ ROLLBACK;
     <listitem>
      <para>
       Any <command>SELECT</command>, <command>INSERT</command>, <command>UPDATE</command>,
-      <command>DELETE</command>, <command>VALUES</command>, <command>EXECUTE</command>,
+      <command>DELETE</command>, <command>MERGE</command>,
+      <command>VALUES</command>, <command>EXECUTE</command>,
       <command>DECLARE</command>, <command>CREATE TABLE AS</command>, or
       <command>CREATE MATERIALIZED VIEW AS</command> statement, whose execution
       plan you wish to see.

doc/src/sgml/ref/prepare.sgml

@@ -116,7 +116,8 @@ PREPARE <replaceable class="parameter">name</replaceable> [ ( <replaceable class
     <listitem>
      <para>
       Any <command>SELECT</command>, <command>INSERT</command>, <command>UPDATE</command>,
-      <command>DELETE</command>, or <command>VALUES</command> statement.
+      <command>DELETE</command>, <command>MERGE</command>, or <command>VALUES</command>
+      statement.
      </para>
     </listitem>
    </varlistentry>

doc/src/sgml/ref/set_transaction.sgml

@@ -121,7 +121,8 @@ SET SESSION CHARACTERISTICS AS TRANSACTION <replaceable class="parameter">transa
    The transaction isolation level cannot be changed after the first query or
    data-modification statement (<command>SELECT</command>,
    <command>INSERT</command>, <command>DELETE</command>,
-   <command>UPDATE</command>, <command>FETCH</command>, or
+   <command>UPDATE</command>, <command>MERGE</command>,
+   <command>FETCH</command>, or
    <command>COPY</command>) of a transaction has been executed.  See
    <xref linkend="mvcc"/> for more information about transaction
    isolation and concurrency control.
@@ -131,8 +132,9 @@ SET SESSION CHARACTERISTICS AS TRANSACTION <replaceable class="parameter">transa
    The transaction access mode determines whether the transaction is
    read/write or read-only.  Read/write is the default.  When a
    transaction is read-only, the following SQL commands are
-   disallowed: <literal>INSERT</literal>, <literal>UPDATE</literal>,
-   <literal>DELETE</literal>, and <literal>COPY FROM</literal> if the
+   disallowed: <command>INSERT</command>, <command>UPDATE</command>,
+   <command>DELETE</command>, <command>MERGE</command>, and
+   <command>COPY FROM</command> if the
    table they would write to is not a temporary table; all
    <literal>CREATE</literal>, <literal>ALTER</literal>, and
    <literal>DROP</literal> commands; <literal>COMMENT</literal>,
@@ -169,7 +171,8 @@ SET SESSION CHARACTERISTICS AS TRANSACTION <replaceable class="parameter">transa
    start of a transaction, before the first query or
    data-modification statement (<command>SELECT</command>,
    <command>INSERT</command>, <command>DELETE</command>,
-   <command>UPDATE</command>, <command>FETCH</command>, or
+   <command>UPDATE</command>, <command>MERGE</command>,
+   <command>FETCH</command>, or
    <command>COPY</command>) of the transaction.  Furthermore, the transaction
    must already be set to <literal>SERIALIZABLE</literal> or
    <literal>REPEATABLE READ</literal> isolation level (otherwise, the snapshot