Missing documentation for dateutil, numpy, and pandas extras

[Zac-HD]

See here. I suspect that this is something to do with the automodule Sphinx directive. We should:

1. Fix this. Obviously, missing documentation is bad.
2. Work out how to test that it doesn't happen again (in the simple case, check that the fully-qualified names appear in the generated pages). This is optional, if we decide to go for (3) directly.
3. Get upstream (ie Sphinx) to add a warning, which is emitted when autowhatever directives produce no output. This would be generally useful, and turn into an error under our CI.

[Zac-HD]

Works on my machine - ie docs are produced as expected - which is going to make debugging "fun".

#1250 might help, or at least listing the correct things to document can't hurt.

I still have no idea of the root cause, but decorators are a perennial issue, as are things imported from another module, and imported decorators are especially bad for Sphinx. Could be that? (except it does work on my machine!) Missing docstring for extra.numpy.boolean_dtypes certainly doesn't help either.