Support defining specific :private-members: for autodoc

[jtpavlock]

Is your feature request related to a problem? Please describe.
Currently, if I'm using autodoc, the :private-members: option does not allow specification of which private members to document. The current behavior is to document all private members, but what if I would only like to document 1 or 2?

Describe the solution you'd like
For :private-members: to take arguments, similarly to how :members: currently works

Describe alternatives you've considered
The current best way to do this is to explicitly list each class in a module and use :autoattribute:

• Some prior discussion: Sphinx throws an "unexpected indentation" error for abstract methods #8009

[jtpavlock]

Wow, thanks for the quick fix!