Doc fix thanks to @jabapyth (#206)

AUTHORS

@@ -142,3 +142,4 @@ that much better:
  * lcya86 刘春洋
  * Martin Alderete (https://github.com/malderete)
  * Nick Joyce
+ * Jared Forsyth

mongoengine/queryset/queryset.py

@@ -467,7 +467,8 @@ class QuerySet(object):
 
     def with_id(self, object_id):
         """Retrieve the object matching the id provided.  Uses `object_id` only
-        and raises InvalidQueryError if a filter has been applied.
+        and raises InvalidQueryError if a filter has been applied. Returns
+        `None` if no document exists with that id.
 
         :param object_id: the value for the id of the document to look up