[clang] [BoundsSafety] Initial documentation for -fbounds-safety (PR #70749)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

FWIW, there were other comments from a previous round of review that didn't get addressed (GitHub may have helpfully hidden them from view for you -- I had to click on a "More" button to get them to show for me.)

https://github.com/llvm/llvm-project/pull/70749