Update manual_is_variant_and documentation to include equality comparison

[krikera]

Update manual_is_variant_and documentation to include equality comparison patterns

This commit updates the documentation for the manual_is_variant_and lint to include all linted cases. Previously, the documentation only mentioned the .map(f).unwrap_or_default() pattern, but the lint also catches equality comparison patterns like option.map(f) == Some(true) and result.map(f) == Ok(true).

changelog: [manual_is_variant_and]: Update documentation to include equality comparison patterns

fixes #15217

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[blyxyas]

Alexendoo's a little bit over the top on capacity (32 assigned PRs)
I'll shave off his last uncommented PRs.

r? @blyxyas

[blyxyas]

LGTM, thanks! ❤️ Also, great that these will be together in the same stable release (I think that the extension isn't yet on stable, right? I had to use master to test these because the stable Clippy wasn't recognizing. these as lintable)

Could you squash these two commits into one?

[krikera]

Sure ! Thanks.