Add examples and/or usage description of `autosummary_filename_map`

[HealthyPear]

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

My problem is essentially described by this comment that I left in the repo of sphinx.automodapi https://github.com/astropy/sphinx-automodapi/issues/73#issuecomment-1466259565

I am not sure if the issue is fixable upstream (aka here) or there, so I chose to bother everyone :)

I kind of understand what the autosummary_filename_map configuration option does, but it is not clear to me if it is supposed to fix my problem and how to properly use it.

Describe the solution you'd like

I'd ask at least for some examples in the documentation (perhaps even inspired by my use case and/or the sciki-learn one.

[HealthyPear]

This might also be related to #1495.

In there the subject is a case in which one has MyClass.my_method() and MyClass.my_Method().

In my case I have instead module.class.my_method() where module is blabla and class is BlaBla.