Improve docs on fix safety for FAST002

[AlexWaygood]

The fix for FAST002 is marked as always unsafe, but there isn't a ## Fix safety section in the documentation explaining why it is marked as unsafe.

I can think of an obvious reason why the fix might be unsafe on Python 3.8 and lower: the fix adds imports of typing_extensions on Python 3.8 and lower, while the user's code might not have typing_extensions listed as a dependency. However, it's not immediately obvious to me (as a non-expert when it comes to fastAPI) what the exact reasons are for the fix being marked as unsafe on Python 3.9+.

It would be great to add better docs here. Cc. @TomerBin, who implemented the rule in #11579 and @charliermarsh, who merged it.

[TomerBin]

At the time it was merged, we weren't handling defaults correctly. If I remember correctly, an improvement was deployed later so we need to check if it's still the case.
This is the relevant fix
#13133