Update the CLR ABI document for vector types

[CarolEidt]

https://github.com/dotnet/runtime/blob/main/docs/design/coreclr/botr/clr-abi.md

As discussed here: dotnet/coreclr#23675 (comment), we need to update the ABI document to clarify the handling of vector types.
Also, once we enable interop for blittable generic types (#23899), we will need to ensure clarity around how these types (i.e. Vector<T>, Vector64<T>, Vector128<T> and Vector256<T>) are handled wrt interop, and to clarify that the existing interop types (Vector2, Vector3 and Vector4) are not actually treated as vectors, but rather as normal structs of N floats, as that is how they are defined.

category:documentation
theme:vector-codegen
skill-level:intermediate
cost:small