Recommend --optimize over --rebuild

It usualy brings same performance improvements in much smaller time.
Signed-off-by: Michal Čihař <michal@cihar.com>

docs/faq.rst

@@ -275,7 +275,14 @@ Fulltext search is too slow
 
 Depending on various conditions (frequency of updates, server restarts and
 other), fulltext index might get too fragmented over time. It is recommended to
-rebuild it from scratch time to time:
+optimize it time to time:
+
+.. code-block:: sh
+
+    ./manage.py rebuild_index --optimize
+
+In case it does not help (or if you have removed lot of strings) it might be
+better to rebuild it from scratch:
 
 .. code-block:: sh