[clang] [BoundsSafety][doc] Make it clear that the feature is work-in-progress (PR #95964)

[Aaron Ballman via cfe-commits]

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

Thank you for updating the documentation, the changes LGTM (had one minor suggestion you can take or leave as you'd like). I think it's better for us to have the documentation than not, so I'm fine with this approach of letting people know about the current and future flags.

https://github.com/llvm/llvm-project/pull/95964