…#10863)

Refs astral-sh#3172 

## Summary

Fix a typo in the docs example, and add a test for the case where a
negative pattern and a positive pattern overlap.

The behavior here is simple: patterns (positive or negative) are always
additive if they hit (i.e. match for a positive pattern, don't match for
a negated pattern). We never "un-ignore" previously-ignored rules based
on a pattern (positive or negative) failing to hit.

It's simple enough that I don't really see other cases we need to add
tests for (the tests we have cover all branches in the ignores_from_path
function that implements the core logic), but open to reviewer feedback.

I also didn't end up changing the docs to explain this more, because I
think they are accurate as written and don't wrongly imply any more
complex behavior. Open to reviewer feedback on this as well!

After some discussion, I think allowing negative patterns to un-ignore
rules is too confusing and easy to get wrong; if we need that, we should
add `per-file-selects` instead.

## Test Plan

Test/docs only change; tests pass, docs render and look right.

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Co-authored-by: Alex Waygood <Alex.Waygood@gmail.com>