[`pyupgrade`] Add fix safety section to docs (`UP029`) (#17490)

## Summary

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

crates/ruff_linter/src/rules/pyupgrade/rules/unnecessary_builtin_import.rs

@@ -27,6 +27,21 @@ use crate::{AlwaysFixableViolation, Fix};
 /// str(1)
 /// ```
 ///
+/// ## Fix safety
+/// This fix is marked as unsafe because removing the import
+/// may change program behavior. For example, in the following
+/// situation:
+///
+/// ```python
+/// def str(x):
+///     return x
+///
+///
+/// from builtins import str
+///
+/// str(1)  # `"1"` with the import, `1` without
+/// ```
+///
 /// ## References
 /// - [Python documentation: The Python Standard Library](https://docs.python.org/3/library/index.html)
 #[derive(ViolationMetadata)]