message: Update nest.flatten documentation to mention that attrs classes are flattened/unflattened.
  See
diff: @@ -234,7 +234,8 @@ def is_nested(seq):
  def flatten(structure, expand_composites=False):
  """Returns a flat list from a given nested structure.
  - If nest is not a sequence, tuple, or dict, then returns a single-element list:
  + If nest is not a sequence, tuple (or a namedtuple), dict, or an attrs class,
  + then returns a single-element list:
  [nest].
  In the case of dict instances, the sequence consists of the values, sorted by
  