[FastAPI] Document fix safety for FAST001 (#22655)

<!--
Thank you for contributing to Ruff/ty! To help us out with reviewing,
please consider the following:

- Does this pull request include a summary of the change? (See below.)
- Does this pull request include a descriptive title? (Please prefix
with `[ty]` for ty pull
  requests.)
- Does this pull request include references to any relevant issues?
-->

## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

## Test Plan

<!-- How was it tested? -->

crates/ruff_linter/src/rules/fastapi/rules/fastapi_redundant_response_model.rs

@@ -58,6 +58,11 @@ use crate::{AlwaysFixableViolation, Fix};
 /// async def create_item(item: Item) -> Item:
 ///     return item
 /// ```
+///
+/// ## Fix safety
+/// This fix is always unsafe, as removing the `response_model` argument can change
+/// runtime behavior and API documentation generation. Additionally, comments inside
+/// the decorator might be removed when the argument is deleted.
 #[derive(ViolationMetadata)]
 #[violation_metadata(stable_since = "0.8.0")]
 pub(crate) struct FastApiRedundantResponseModel;