Added comment about $all's inefficiency

See issue https://jira.mongodb.org/browse/SERVER-1000

Author: giveturtle

source/reference/operator/all.txt

@@ -53,6 +53,13 @@ $all
 
       In most cases, MongoDB does not treat arrays as sets. This
       operator provides a notable exception to this approach.
+
+      Currently, $all queries scan all the documents that match
+      the first element in the query array. This means that even with
+      when the field is indexed, queries may take a very long time
+      to return (especially when the first element in the array appears
+      in many documents).
+      See: https://jira.mongodb.org/browse/SERVER-1000
 
    .. seealso:: 
       :method:`find() <db.collection.find()>`, :method:`update()