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

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

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Co-authored-by: Ahmed Bougacha <[email protected]>
Co-authored-by: Akira Hatanaka <[email protected]>
Co-authored-by: John Mccall <[email protected]>

Author: 4 people