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-message.el

@@ -344,5 +344,6 @@ Emacs. Based on code by Titus von der Malsburg."
        (erase-buffer)
        (call-process-shell-command mu4e-html2text-command tmp-file t t)
        (delete-file tmp-file))) msg))
+
 ;;; _
 (provide 'mu4e-message)