Have option to emit Sphinx warnings if no docstring is found

[astrofrog]

I originally implemented this in #103 but in the mean time, we've changed to no longer use the AstropyAutosummary class. I've opened this issue to keep track of this wishlist item nevertheless. Maybe we should propose this option in the upstream autosummary?

[embray]

I think submitting a PR to sphinx would be the best approach. In the meantime there's no reason we can't switch back to using a subclass, though I'd say only if it's easy to implement this functionality in a subclass in some way that doesn't require copying any code from the base implementation (the fact that we were doing this previously ran afoul of updates to Sphinx).

[astrofrog]

This issue has been moved to astropy/sphinx-automodapi#45