Add an option to emit warnings if public objects don't have a docstring

As originally mentioned in https://github.com/astropy/astropy-helpers/issues/195, it might be useful to have a way to be strict about requiring public functions, methods, and classes to have docstrings, so that we don't end up with empty docstrings in the API docs generated by sphinx-automodapi.

Start by reading the linked astropy-helpers issue and tracing how sphinx-automodapi identifies public functions, methods, and classes before generating API documentation. Define done as an opt-in setting that reports public objects without docstrings and prevents silent empty entries in the generated Sphinx documentation.

Written by the indexing model from the issue text.