Adapt `doc-valid-idents` config to support list expansion instead of replacement

[xFrednet]

Description

The config value doc-valid-idents for doc_markdown is a list of allowed identifiers. I've seen some projects overriding them and only adding a few new values. It would be nice if the user also had the option to append new values to the list instead of replacing them.

I was thinking of something like a dummy value .. that indicates that the default list should only be expanded. For example:

doc-valid-idents = ["OverRide"] # => Only `OverRide` is allwed 
doc-valid-idents = ["..", "Append"] # => Clippy's default plus override is allowed

This is just one suggestion, feel free to suggest other options :)

Version

Any *

Additional Labels

No response

[xFrednet]

This should hopefully be simple (Famous last words xD)

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