Doc: add a section in Part II concerning RETURNING.

There are assorted references to RETURNING in Part II, but nothing
that would qualify as an explanation of the feature, which seems
like an oversight considering how useful it is.  Add something.

Noted while looking for a place to point a cross-reference to ...

doc/src/sgml/queries.sgml

@@ -2262,7 +2262,8 @@ SELECT * FROM moved_rows;
 
    <para>
     Data-modifying statements in <literal>WITH</> usually have
-    <literal>RETURNING</> clauses, as seen in the example above.
+    <literal>RETURNING</> clauses (see <xref linkend="dml-returning">),
+    as shown in the example above.
     It is the output of the <literal>RETURNING</> clause, <emphasis>not</> the
     target table of the data-modifying statement, that forms the temporary
     table that can be referred to by the rest of the query.  If a