[clang] [LifetimeSafety] Add user documentation (PR #183058)

[Gábor Horváth via cfe-commits]

https://github.com/Xazax-hun approved this pull request.

Overall, looks good to me. A couple of more nits inline. Feel free to merge once those are addressed.

https://github.com/llvm/llvm-project/pull/183058