doc: update storage description in config reference

doc/reference.rst

@@ -810,7 +810,7 @@ Statement ``query_module`` takes a list of ``module_name
 
 Data directory for zones.  It is used to store zone files and journal
 files. If compiled with LMDB support, a database storing persistent zone
- event timers will also be created in the ``timers`` subdirectory.
+event timers for slave zones will be created in the ``timers`` subdirectory.
 
 Value of ``storage`` set in ``zone`` section is relative to
 ``storage`` in ``zones`` section.