Document that an SQL-standard-compliant query that expects a unique row

from information_schema.referential_constraints might return several in
Postgres because we allow duplicate-named constraints in the same
schema.

doc/src/sgml/information_schema.sgml

@@ -3211,6 +3211,19 @@ ORDER BY c.ordinal_position;
     </tbody>
    </tgroup>
   </table>
+
+  <note>
+   <para>
+    The SQL standard requires constraint names to be unique within a
+    schema;  <productname>PostgreSQL</productname>, however, does not
+    enforce this restriction.  If duplicate-named constraints are
+    stored in the same <productname>PostgreSQL</productname> schema, a
+    standard-compliant query that expects to return one row might
+    return several, one for each matching constraint stored in the
+    specified schema.
+   </para>
+  </note>
+
  </sect1>
 
  <sect1 id="infoschema-role-column-grants">