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

[Oliver Hunt via cfe-commits]

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@@ -163,14 +163,20 @@ a cryptographic signature, other implementations may be possible.  See
     data is simply a pepper added to the hash, not an encryption key, and so
     can be initialized using random data.
 
+ *** FIXME *** fpac?
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ojhunt wrote:

yup, I would _swear_ I stripped those out after I got that all written up

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