[clang] [clang][ptrauth] Warn about the use of a weak signing schema (PR #157779)

[Martin Balao Alonso via cfe-commits]

martinuy wrote:

I pushed a couple of doc changes based on the last discussions we had in this PR.

Please let me know if you have any other concern, request or if it looks good to you.

https://github.com/llvm/llvm-project/pull/157779