[NFC][Clang][Docs] Update Pointer Authentication documentation

This updates the pointer authentication documentation to include
a complete description of the existing functionaliy and behaviour,
details of the more complex aspects of the semantics and security
properties, and the Apple arm64e ABI design.

Co-authored-by: Ahmed Bougacha
Co-authored-by: Akira Hatanaka
Co-authored-by: John Mccall

Author: ojhunt