mu4e: Place provide forms in their own sections

Use "_" as the title of that section so that it is less distracting
when sections are collapsed to get an overview of the library.

Using a separate section is useful because it reduces the risk of
accidentally into the middle of a library.

mu4e/mu4e-main.el

@@ -287,5 +287,6 @@ clicked."
           (sit-for 1)
           (mu4e~main-menu))))))
 
+;;; _
 (provide 'mu4e-main)
 ;;; mu4e-main.el ends here