doc: move the mention of aggregate JSON functions up in section

It was previously easily overlooked at the end of several tables.

Reported-by: Alex Denman

Discussion: https://postgr.es/m/166335888474.659.16897487975376230364@wrigleys.postgresql.org

Backpatch-through: 10

doc/src/sgml/func.sgml

@@ -15149,6 +15149,12 @@ table2-mapping
    <type>jsonb</type>, though not for <type>json</type>.  The comparison
    operators follow the ordering rules for B-tree operations outlined in
    <xref linkend="json-indexing"/>.
+   See also <xref linkend="functions-aggregate"/> for the aggregate
+   function <function>json_agg</function> which aggregates record
+   values as JSON, the aggregate function
+   <function>json_object_agg</function> which aggregates pairs of values
+   into a JSON object, and their <type>jsonb</type> equivalents,
+   <function>jsonb_agg</function> and <function>jsonb_object_agg</function>.
   </para>
 
   <table id="functions-json-op-table">
@@ -16604,15 +16610,6 @@ table2-mapping
      </tbody>
     </tgroup>
    </table>
-
-  <para>
-    See also <xref linkend="functions-aggregate"/> for the aggregate
-    function <function>json_agg</function> which aggregates record
-    values as JSON, the aggregate function
-    <function>json_object_agg</function> which aggregates pairs of values
-    into a JSON object, and their <type>jsonb</type> equivalents,
-    <function>jsonb_agg</function> and <function>jsonb_object_agg</function>.
-  </para>
  </sect2>
 
  <sect2 id="functions-sqljson-path">