Docs: Document the exception in no-unsafe-negation

[mdjermanovic]

What is the purpose of this pull request? (put an "X" next to item)

[X] Documentation update

What changes did you make? (Give an overview)

no-unsafe-negation allows parenthesized negations, e.g.:

/*eslint no-unsafe-negation: "error"*/

if ((!a) in b) {} // no errors

It's currently undocumented.

This PR documents that as an 'exception'.

Is there anything you'd like reviewers to focus on?

Technically, this might not be 'negating the left operand', which the rule disallows, so this wouldn't be an 'exception', but I think it should be documented either way.

[platinumazure]

LGTM, thanks!

[kaicataldo]

LGTM, thanks

[kaicataldo]

Thanks for contributing to ESLint!