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/dml.sgml

@@ -102,6 +102,18 @@ INSERT INTO products (product_no, name, price) VALUES
 </programlisting>
   </para>
 
+  <para>
+   It is also possible to insert the result of a query (which might be no
+   rows, one row, or many rows):
+<programlisting>
+INSERT INTO products (product_no, name, price)
+  SELECT product_no, name, price FROM new_products
+    WHERE release_date = 'today';
+</programlisting>
+   This provides the full power of the SQL query mechanism (<xref
+   linkend="queries">) for computing the rows to be inserted.
+  </para>
+
   <tip>
    <para>
     When inserting a lot of data at the same time, considering using
@@ -252,4 +264,91 @@ DELETE FROM products;
    then all rows in the table will be deleted!  Caveat programmer.
   </para>
  </sect1>
+
+ <sect1 id="dml-returning">
+  <title>Returning Data From Modified Rows</title>
+
+  <indexterm zone="dml-returning">
+   <primary>RETURNING</primary>
+  </indexterm>
+
+  <indexterm zone="dml-returning">
+   <primary>INSERT</primary>
+   <secondary>RETURNING</secondary>
+  </indexterm>
+
+  <indexterm zone="dml-returning">
+   <primary>UPDATE</primary>
+   <secondary>RETURNING</secondary>
+  </indexterm>
+
+  <indexterm zone="dml-returning">
+   <primary>DELETE</primary>
+   <secondary>RETURNING</secondary>
+  </indexterm>
+
+  <para>
+   Sometimes it is useful to obtain data from modified rows while they are
+   being manipulated.  The <command>INSERT</>, <command>UPDATE</>,
+   and <command>DELETE</> commands all have an
+   optional <literal>RETURNING</> clause that supports this.  Use
+   of <literal>RETURNING</> avoids performing an extra database query to
+   collect the data, and is especially valuable when it would otherwise be
+   difficult to identify the modified rows reliably.
+  </para>
+
+  <para>
+   The allowed contents of a <literal>RETURNING</> clause are the same as
+   a <command>SELECT</> command's output list
+   (see <xref linkend="queries-select-lists">).  It can contain column
+   names of the command's target table, or value expressions using those
+   columns.  A common shorthand is <literal>RETURNING *</>, which selects
+   all columns of the target table in order.
+  </para>
+
+  <para>
+   In an <command>INSERT</>, the data available to <literal>RETURNING</> is
+   the row as it was inserted.  This is not so useful in trivial inserts,
+   since it would just repeat the data provided by the client.  But it can
+   be very handy when relying on computed default values.  For example,
+   when using a <link linkend="datatype-serial"><type>serial</></link>
+   column to provide unique identifiers, <literal>RETURNING</> can return
+   the ID assigned to a new row:
+<programlisting>
+CREATE TABLE users (firstname text, lastname text, id serial primary key);
+
+INSERT INTO users (firstname, lastname) VALUES ('Joe', 'Cool') RETURNING id;
+</programlisting>
+   The <literal>RETURNING</> clause is also very useful
+   with <literal>INSERT ... SELECT</>.
+  </para>
+
+  <para>
+   In an <command>UPDATE</>, the data available to <literal>RETURNING</> is
+   the new content of the modified row.  For example:
+<programlisting>
+UPDATE products SET price = price * 1.10
+  WHERE price &lt;= 99.99
+  RETURNING name, price AS new_price;
+</programlisting>
+  </para>
+
+  <para>
+   In a <command>DELETE</>, the data available to <literal>RETURNING</> is
+   the content of the deleted row.  For example:
+<programlisting>
+DELETE FROM products
+  WHERE obsoletion_date = 'today'
+  RETURNING *;
+</programlisting>
+  </para>
+
+  <para>
+   If there are triggers (<xref linkend="triggers">) on the target table,
+   the data available to <literal>RETURNING</> is the row as modified by
+   the triggers.  Thus, inspecting columns computed by triggers is another
+   common use-case for <literal>RETURNING</>.
+  </para>
+
+ </sect1>
 </chapter>

doc/src/sgml/queries.sgml

@@ -2259,7 +2259,8 @@ 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