Document `pauthtest`

[asl]

#97237 implemented a way to specify pointer authentication ABI via explicit -mabi option or via target triple environment component. We need to document few aspects here:

• Transient nature of pauthtest. Noone should rely on this for real implementation
• How target would implemented pointer authentication ABI, define signing scheme, etc.

[tru]

@asl can this be removed from LLVM 19.x milestone?