[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 <[email protected]>
Co-authored-By: Akira Hatanaka <[email protected]>
Co-authored-By: John Mccall <[email protected]>

Author: 4 people