[clang] [NFC][BoundsSafety] Give `counted_by_or_null`, `sized_by`, and `sized_by_or_null` their own documentation (PR #212877)

[via cfe-commits]

llvmbot wrote:

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Failed to cherry-pick: 45271fc23fd23f83eace959285abfd580a267a5

https://github.com/llvm/llvm-project/actions/runs/32166999699

Please manually backport the fix and push it to your github fork.  Once this is done, please create a [pull request](https://github.com/llvm/llvm-project/compare)

https://github.com/llvm/llvm-project/pull/212877