Even though the documentation of
hook_menu() doesn't mention it, the
plid attribute can be used for specifying the ID of the parent menu link. You can set it to 0 if you don't want your menu item to have a parent.
It's probably not included in the documentation, because it's problematic to know the menu link ID (
mlid) on that level where you use
hook_menu(). It's an incremental identifier in the database, so there is no guarantee that it will always be the same (e.g. after reinstalling the site if you work on an installation profile). So I don't think it's a good idea to make a use of this attribute, except of course when you have this simple use case that setting it to zero does the job.