[pyupgrade] Add fix safety section to docs (UP029)

[Kalmaegi]

Summary

add fix safety section to UP029: unnecessary_builtin_import.rs, for #15584

I conducted some tests and found that the only side effect should be the removal of inline comments along with the code. However, during the deletion process, comments placed above the code are preserved. I'm not sure if this behavior needs to be explicitly described.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[VascoSch92]

I have two example one should look into :-)

The first one is the case where we import a builtin with an alias, e.g., from builtins import str as builtin_str, is the rule removing this import or not?

For the second, let's suppose that we do something like:

# Original code
def str(x):
    return f"Custom: {x}"

# somewhere in the code
from builtins import str
result = str(1)  # Uses Python's builtin str

then the rules will correct that as

def str(x):
    return f"Custom: {x}"

result = str(1)

which is a change of behaviour.

[Kalmaegi]

I have two example one should look into :-)

The first one is the case where we import a builtin with an alias, e.g., from builtins import str as builtin_str, is the rule removing this import or not?

For the second, let's suppose that we do something like:

# Original code
def str(x):
    return f"Custom: {x}"

# somewhere in the code
from builtins import str
result = str(1)  # Uses Python's builtin str

then the rules will correct that as

def str(x):
    return f"Custom: {x}"

result = str(1)

which is a change of behaviour.

The first point should be skipped, but the second one does affect the original behavior. Thank you very much!