[llvm] [Coroutines][Docs] Add a discussion on the handling of certain parameter attribs (PR #117183)

[Tyler Nowicki via llvm-commits]

TylerNowicki wrote:

@rjmccall Ping, this is documentation for how the parameter attribs ByVal and SwiftError should be handled by the coroutine passes.

https://github.com/llvm/llvm-project/pull/117183