correct use of 'size' param

Author: milkie

source/reference/method/db.createCollection.txt

@@ -20,7 +20,8 @@ db.createCollection()
 
    :param bytes size: If ``capped`` is ``true``, ``size`` specifies a
                       maximum size in bytes for the capped collection.
-                      When ``capped`` is false, you may not use ``size``.
+                      When ``capped`` is false, you may use ``size`` to
+                      preallocate space.
 
    :option autoIndexId: If ``capped`` is ``true`` you can specify
                         ``false`` to disable the automatic index