clarifying the 'push' atomic update docs

the first time I read this I was all like... "no duh it will remove either the first or the last, but which does it do???"

docs/guide/querying.rst

@@ -499,11 +499,13 @@ that you may use with these methods:
 * ``dec`` -- decrement a value by a given amount
 * ``push`` -- append a value to a list
 * ``push_all`` -- append several values to a list
-* ``pop`` -- remove the first or last element of a list
+* ``pop`` -- remove the first or last element of a list `depending on the value`_
 * ``pull`` -- remove a value from a list
 * ``pull_all`` -- remove several values from a list
 * ``add_to_set`` -- add value to a list only if its not in the list already
 
+.. _depending on the value: http://docs.mongodb.org/manual/reference/operator/update/pop/
+
 The syntax for atomic updates is similar to the querying syntax, but the
 modifier comes before the field, not after it::