feature: warn for type alias without docstring

[AntoineD]

Is your feature request related to a problem? Please describe.

Where used, type aliases without docstring are rendered with no link to their definition.

Describe the solution you'd like

Issue a warning when a symbol refers to a type alias with no docstring such that is can be identified.

[pawamoy]

Can you elaborate with examples/snippets/screenshots? I'm not sure to fully understand.

[pawamoy]

I believe it's linters territory: warning users that a public object does not have a docstring. But since mkdocstrings works with docstrings, maybe we could consider adding such a check. This will have to wait for Griffe to have proper warning configuration options 🙂