Add `flake8-fixme` documentation (#5868)
## Summary
Completes documentation for the `flake8-fixme` (`FIX`) ruleset. Related
to #2646.
Tweaks the violation message. For example,
```
FIX001 Line contains FIXME
```
becomes
```
FIX001 Line contains FIXME, consider resolving the issue
```
This is because the previous message was unclear if it was warning
against the use of FIXME tags per se, or the code the FIXME tag was
annotating.
## Test Plan
`cargo test && python scripts/check_docs_formatted.py`
