Fix documentation of information_schema.element_types

The documentation of the columns collection_type_identifier and
dtd_identifier was wrong.  This effectively reverts commits
8e1ccad5 and
57352df6 and updates the name
array_type_identifier (the name in SQL:1999) to
collection_type_identifier.

closes bug #5926

doc/src/sgml/information_schema.sgml

@@ -2159,7 +2159,7 @@
 SELECT c.column_name, c.data_type, e.data_type AS element_type
 FROM information_schema.columns c LEFT JOIN information_schema.element_types e
      ON ((c.table_catalog, c.table_schema, c.table_name, 'TABLE', c.dtd_identifier)
-       = (e.object_catalog, e.object_schema, e.object_name, e.object_type, e.dtd_identifier))
+       = (e.object_catalog, e.object_schema, e.object_name, e.object_type, e.collection_type_identifier))
 WHERE c.table_schema = '...' AND c.table_name = '...'
 ORDER BY c.ordinal_position;
 </programlisting>
@@ -2219,11 +2219,13 @@ ORDER BY c.ordinal_position;
      </row>
 
      <row>
-      <entry><literal>dtd_identifier</literal></entry>
+      <entry><literal>collection_type_identifier</literal></entry>
       <entry><type>sql_identifier</type></entry>
       <entry>
        The identifier of the data type descriptor of the array being
-       described
+       described.  Use this to join with the
+       <literal>dtd_identifier</literal> columns of other information
+       schema views.
       </entry>
      </row>
 
@@ -2378,6 +2380,14 @@ ORDER BY c.ordinal_position;
       <entry>Always null, because arrays always have unlimited maximum cardinality in <productname>PostgreSQL</></entry>
      </row>
 
+     <row>
+      <entry><literal>dtd_identifier</literal></entry>
+      <entry><type>sql_identifier</type></entry>
+      <entry>
+       An identifier of the data type descriptor of the element.  This
+       is currently not useful.
+      </entry>
+     </row>
     </tbody>
    </tgroup>
   </table>