Clarify that the function focus_widget is only used to focus the "staged" and "unstaged" widgets

I was the person who suggested `focus_widget`, but in hindsight, it is probably not a very good name. A reader might think it can be used to focus *any* widget. So, it is a good idea to clarify that it is only used for the "staged" and "unstaged" widgets by either renaming it to something clearer, or adding a comment to clarify. I personally would prefer a rename.
