added use of interoperable with to specialized iterators

[SVN r21701]

doc/transform_iterator_ref.rst

@@ -102,6 +102,11 @@ the ``Iterator`` argument models.
 If ``transform_iterator`` models Writable Lvalue Iterator then it is a
 mutable iterator (as defined in the old iterator requirements).
 
+If iterator ``X`` is interoperable with iterator ``Y`` then
+``transform_iterator<F1, X, R1, V1>`` is interoperable with
+``transform_iterator<F2, Y, R2, V2>``.
+
+
 
 ``transform_iterator`` operations
 .................................