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

[Aaron Ballman via cfe-commits]

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

I don't have much of value to say on the documentation changes themselves, but I definitely appreciate making the docs easier for users, so thank you!

I'm approving but please don't land until one of the bounds safety folks verifies the correctness of the final content. I didn't spot any issues, but I am pretty sure I also approved the original docs, sooooooo. :-)

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