Update online doc for output storage.

doc/extending/op.txt

@@ -74,7 +74,8 @@ following methods:
 
       A function Mode may allow ``output_storage`` elements to persist between
       evaluations, or it may reset ``output_storage`` cells to hold a value of
-      ``None``.  This feature can allow ``perform`` to reuse memory between
+      ``None``.  It can also pre-allocate some memory for the Op to use.
+      This feature can allow ``perform`` to reuse memory between
       calls, for example.
 
   This method must be determined by the inputs. That is to say, if