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

[Tyler Nowicki via llvm-commits]

================
@@ -810,6 +810,28 @@ The LLVM IR for a coroutine using a Coroutine with a custom ABI looks like:
     ret ptr %hdl
   }
 
+Parameter Attributes
+====================
+Some parameter attributes, used to communicate additional information about the result or parameters of a function, require special handling.
+
+ByVal
+-----
+A ByVal parameter on an argument indicates that the pointer parameter should really be passed by value to the function.
----------------
TylerNowicki wrote:

```suggestion
A ByVal parameter on an argument indicates that the pointee should be treated as being passed by value to the function.
```

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