Make InheritDocstrings work for __call__

At the moment, InheritDocstrings ignores any hidden method:

https://github.com/astropy/astropy/blob/master/astropy/utils/misc.py#L497

which means ``__call__``, but it would be nice for that to work since ``__call__`` can appear in API documentation. Can we add an exception for ``__call__``?