[pylint] Preserve trailing comments in useless-return fix (PLR1711) - #29180

Summary

Closes #28861

The useless-return rule (PLR1711) removes the final return / return None
in a function. When that line had a trailing comment, the fix deleted the whole
line and the comment was lost — and the fix was still marked safe.

This keeps a trailing comment on its own line instead of deleting it, the same
way PIE790 and PYI013 already do. If removing the statement would still drop
a comment (for example a comment inside a parenthesized multi-line return), the
fix is now marked unsafe rather than silently deleting it. I also added a "Fix
safety" section to the rule docs.

One thing I wasn't sure about: pragma-style trailing comments like # noqa or
# fmt: off can change meaning when moved onto their own line. I left them as
ordinary comments for now (matching PIE790/PYI013), but happy to special-case
them if you'd prefer.

Test Plan

Added fixture cases to useless_return.py: the example from the issue, a method
with return None # ..., a return; with a comment, a same-line
print(...); return # ..., and a multi-line parenthesized return (the unsafe
case). The existing # [useless-return] comments now survive in the snapshot.
cargo test -p ruff_linter, clippy, and the formatting hooks all pass.