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

[Mehdi Amini via llvm-commits]

joker-eph wrote:

@efriedma-quic  merging this within 24h while a discussion is on-going seems like not the usual norm in the community.

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