[llvm-branch-commits] [clang] 🍒[NFC][BoundsSafety] Give `counted_by_or_null`, `sized_by`, and `sized_by_or_null` their own documentation (#212877) (PR #217720)

[Oliver Hunt via llvm-branch-commits]

https://github.com/ojhunt approved this pull request.

This is just upstreaming docs, so I believe this is reasonable for the release branch

https://github.com/llvm/llvm-project/pull/217720