Allow for slots to be documented in the 3.8 mentioned dictionary style

[13steinj]

Is your feature request related to a problem? Please describe.
To an extent. Sphinx's support of slots and conflicts between slots and the ideas of member types vs class variables has been a long standing issue (see context).

Describe the solution you'd like
If slots is a dictionary, the values of the dictionary can be used as docstrings for that slot.

Describe alternatives you've considered
Current documentation methods for slots, but it is lacking and has issues with conflicts and more.

Additional context

• https://docs.python.org/dev/whatsnew/3.8.html#inspect
• Behavior when documenting a __slots__ attribute is unexpected #4678
• Behavior when documenting a __slots__ attribute is unexpected #4678 (comment)
• PythonDomain.resolve_xref incorrectly matches ivars to class methods #2549 (comment)

[tk0miya]

Done in #6388.
Thank you for reporting!