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

[Dan Liew via cfe-commits]

delcypher wrote:

@AaronBallman 

Thanks for the review.

> 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. :-)

I am one of the bounds safety folks and so is @apple-fcloutier so I guess this is good to go :)

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