message: Add example to map_structure_up_to's docstring that clarifies that shallow_tree can be smaller than input.
diff: @@ -626,7 +626,7 @@ def assert_shallow_structure(shallow_tree, input_tree, check_types=True):
  The following code will not raise an exception:
  ```python
  shallow_tree = ["a", "b"]
  - input_tree = ["c", ["d", "e"]]
  + input_tree = ["c", ["d", "e"], "f"]
  assert_shallow_structure(shallow_tree, input_tree)
  ```
  @@ -875,6 +875,14 @@ def map_structure_up_to(shallow_tree, func, *inputs, **kwargs):
  Examples:
  + ```python
  + shallow_tree = [None, None]
  + inp_val = [1, 2, 3]
  + out = map_structure_up_to(shallow_tree, lambda x: 2 * x, inp_val)
  +
  + # Output is: [2, 4]
  + ```
  +
  ```python
  ab_tuple = collections.namedtuple("ab_tuple", "a, b")
  op_tuple = collections.namedtuple("op_tuple", "add, mul")
  