Improve docs for clippy_utils's MSRV support

[BD103]

This PR adds documentation to extract_msrv_attr!, improves the documentation on Msrv and MsrvStack, and slightly refactors Msrv::read_cargo(). While I've used clippy_utils before, this is my first time interacting with the MSRV side of it, so please check over my writing to make sure it is correct!

Let me know if there's anything else I need to do. :)

changelog: none

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[ada4a]

This is awesome, thank you very much:) I remember spending a non-trivial amount of time trying to figure out what the weird extract_msrv_attr!() call on the bottom of EarlyLintPasses was.

Left just a couple of wording nits

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[BD103]

Thanks for the feedback! Let me know if you want me to squash my changes together.

[ada4a]

I think that'd make sense, yeah:) But note that I won't be able to actually approve the changes afterwards, as I'm not a team member^^