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-mark.el

@@ -459,5 +459,6 @@ action', return nil means 'don't do anything'."
        (when (eq what 'apply)
          (mu4e-mark-execute-all t))))))
 
+;;; _
 (provide 'mu4e-mark)
 ;;; mu4e-mark.el ends here