docs: add callout box for unintuitive behavior

[benmccann]

Prerequisites checklist

• I have read the contributing guidelines.

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

[x] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofix to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

What changes did you make? (Give an overview)

closes #18562

I found this behavior to be very surprising and numerous maintainers also agreed in #17213

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

I don't know if there are other callout boxes you use besides "important". I saw that used on another page, so used the same one, but perhaps there's a more appropriate one?

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[nzakas]

Simple enough. LGTM.