[pyupgrade] Document more fix unsafety for UP007

[hauntsaninja]

from typing import Optional
x = "asdf"
def foo(a: Optional[x]):
    pass

It's not uncommon to see issues like this with multiprocessing, where things are actually methods instead of types

Relates to #8482

[github-actions]

ruff-ecosystem results

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[charliermarsh]

Thanks!