doc: fix description of backward_scan

RhodiumToad · RhodiumToad · commit ee68541ce60d · 2018-05-17T21:23:01.000+01:00
The description of the index property backward_scan was incorrect and
misleading; rectify.

Backpatch to 9.6 where the amutils functionality was introduced.
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
@@ -17366,7 +17366,9 @@ 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>
