Fix doc omission for MERGE into updatable views.

Commit 5f2e179bd3 missed one place in rules.sgml that should have
mentioned MERGE. Also, be more specific when saying that MERGE doesn't
support rules, since it does support SELECT rules.

doc/src/sgml/rules.sgml

@@ -859,7 +859,8 @@ SELECT t1.a, t2.b, t1.ctid FROM t1, t2 WHERE t1.a = t2.a;
     a view. These rules will rewrite the command, typically into a command
     that updates one or more tables, rather than views. That is the topic
     of <xref linkend="rules-update"/>.  Note that this will not work with
-    <command>MERGE</command>, which currently does not support rules.
+    <command>MERGE</command>, which currently does not support rules on
+    the target relation other than <command>SELECT</command> rules.
 </para>
 
 <para>
@@ -873,7 +874,8 @@ SELECT t1.a, t2.b, t1.ctid FROM t1, t2 WHERE t1.a = t2.a;
 
 <para>
     Automatic rewriting of an <command>INSERT</command>,
-    <command>UPDATE</command>, or <command>DELETE</command> query on a
+    <command>UPDATE</command>, <command>DELETE</command>, or
+    <command>MERGE</command> query on a
     simple view is always tried last. Therefore, if a view has rules or
     triggers, they will override the default behavior of automatically
     updatable views.