doc: fix description of backward_scan

The description of the index property backward_scan was incorrect and
misleading; rectify.

Backpatch to 9.6 where the amutils functionality was introduced.

doc/src/sgml/func.sgml

@@ -17519,7 +17519,9 @@ SELECT currval(pg_get_serial_sequence('sometable', 'id'));
      </row>
      <row>
       <entry><literal>backward_scan</literal></entry>
-      <entry>Can the index be scanned backwards?
+      <entry>Can the scan direction be changed in mid-scan (to
+             support <literal>FETCH BACKWARD</literal> on a cursor without
+             needing materialization)?
       </entry>
      </row>
     </tbody>