Update the comment for `TypesUtils.ty_is_copyable`

[sonmarcho]

https://github.com/AeneasVerif/charon/blob/a0c861ca28c34c72e43b55467f09bbfbafc7fb20/charon-ml/src/TypesUtils.ml#L209

The comment should explain why we keep the function:

• it is used in the code of Aeneas exactly where we need to check that a type is copyable
• for now we don't do the check (as the function always returns true) but we might want to do something more precise in the future.
• if we do so, it is good not to have to dive in the code to notice where we should reinsert the checks, because we would likely miss some