This is mostly just cleanup:
1. Removes `diagnostic-descriptions.md` since it isn't used any more
2. Adds the group name to all the old notes files
3. Removes trailing whitespace
4. Adds "See Also" sections for notes that have links
Apply suggestions from code review
Co-authored-by: Ben Rimmington <me@benrimmington.com>
Reword explanation of pre-5.3 behavior
Associate SE-0286 note with warning diagnostic