Modules should inherit top-comments from hidden includes too

[lpw25]

#654 made it so that an include that is marked with (** @inline *) can import a top-comment into a module if it's the first item. I think it would be good to extend this to also work for includes of hidden modules. Such includes are already automatically expanded, and commonly occur when people use the _intf.ml pattern because the _intf module is not usually exposed by the library. I think this would save people from a lot of @inline annotation, or more realistically, improve a lot of documentation where people have neglected to do that.