[clang] [NFC][Clang][Docs] Update Pointer Authentication documentation (PR #152596)

[Anton Korobeynikov via cfe-commits]

https://github.com/asl commented:

Looks great to me! Couple of minor things.

Tagging @atrosinenko @kbeyls @smithp35 for visibility

https://github.com/llvm/llvm-project/pull/152596