Summary

Currently the name of PYI019 is custom-type-var-return-type and its diagnostic message is:

"Methods like {method_name} should return Self instead of a custom TypeVar"

There are two issues that this PR addresses:

• Often if you're converting a method to use Self rather than a custom TypeVar, you'll need to change other annotations in the method's non-self parameters, not just the self annotation and the return annotation. Indeed, our preview-mode autofix for stub files will change some of these annotations. But the message and name imply that the rule is just about the return annotation.
• It should be pretty obvious which method the diagnostic is complaining about from the diagnostic's range, so I don't think we really need to mention the method name in the diagnostic message. It's more useful to mention the name of the TypeVar that we're suggesting the user should get rid of.

I think the changes here should all be backwards-compatible, but we'll need to remember to add a redirect for the documentation after the next release (whether it is a minor release or patch release). In my opinion, the diagnostic's range should also cover the whole of the function's header, but this would be backwards-incompatible, so I'll propose it separately (it will either need to be preview-only or wait until the next minor release).

Test Plan

cargo test -p ruff_linter --lib