[llvm] [LangRef] Add a description of the semantics of call signatures. (PR #136189)

[Mehdi Amini via llvm-commits]

https://github.com/joker-eph requested changes to this pull request.

I don't quite get it: #63484 calls this a bug and a unintended regression of the opaque pointer migration.
I don't understand why it is proper to endorse this in LangRef instead of fixing the verifier.

https://github.com/llvm/llvm-project/pull/136189