Doc: improve examples for json_populate_record() and related functions.

Make these examples self-contained by providing declarations of the
user-defined row types they rely on.  There wasn't room to do this
in the old doc format, but now there is, and I think it makes the
examples a good bit less confusing.

Author: tglsfdc

doc/src/sgml/func.sgml

@@ -15387,7 +15387,12 @@ table2-mapping
         calls.
        </para>
        <para>
-        <literal>select * from json_populate_record(null::myrowtype, '{"a": 1, "b": ["2", "a b"], "c": {"d": 4, "e": "a  b c"}}')</literal>
+        <literal>create type subrowtype as (d int, e text);</literal>
+        <literal>create type myrowtype as (a int, b text[], c subrowtype);</literal>
+       </para>
+       <para>
+        <literal>select * from json_populate_record(null::myrowtype,
+         '{"a": 1, "b": ["2", "a b"], "c": {"d": 4, "e": "a  b c"}, "x": "foo"}')</literal>
         <returnvalue></returnvalue>
 <programlisting>
  a |   b       |      c
@@ -15419,7 +15424,10 @@ table2-mapping
         for <function>json[b]_populate_record</function>.
        </para>
        <para>
-        <literal>select * from json_populate_recordset(null::myrowtype, '[{"a":1,"b":2},{"a":3,"b":4}]')</literal>
+        <literal>create type twoints as (a int, b int);</literal>
+       </para>
+       <para>
+        <literal>select * from json_populate_recordset(null::twoints, '[{"a":1,"b":2},{"a":3,"b":4}]')</literal>
         <returnvalue></returnvalue>
 <programlisting>
  a | b
@@ -15456,7 +15464,10 @@ table2-mapping
         input record value, unmatched columns are always filled with nulls.
        </para>
        <para>
-        <literal>select * from json_to_record('{"a":1,"b":[1,2,3],"c":[1,2,3],"e":"bar","r": {"a": 123, "b": "a b c"}}') as x(a int, b text, c int[], d text, r myrowtype) </literal>
+        <literal>create type myrowtype as (a int, b text);</literal>
+       </para>
+       <para>
+        <literal>select * from json_to_record('{"a":1,"b":[1,2,3],"c":[1,2,3],"e":"bar","r": {"a": 123, "b": "a b c"}}') as x(a int, b text, c int[], d text, r myrowtype)</literal>
         <returnvalue></returnvalue>
 <programlisting>
  a |    b    |    c    | d |       r