emacs: Add instructions to the hidden citations/signatures.

We've received a user report that the hidden citations were annoying
since the user couldn't tell what was being referred to by subsequent
text. Apparently it wasn't obvious enough that the hidden citation
could be revealed by clicking or by pressing Enter. So make the button
text say as much.

notmuch.el

@@ -613,13 +613,13 @@ which this thread was originally shown."
               (let ((p (point-marker))
                     (cite-button-text
                      (concat "["  (number-to-string (count-lines beg-sub (point)))
-                             "-line citation.]")))
+                             "-line citation. Click/Enter to show.]")))
                 (goto-char (- beg-sub 1))
                 (insert (concat "\n" indent))
                 (insert-button cite-button-text
                                'invisibility-spec invis-spec
                                :type 'notmuch-button-citation-toggle-type)
-                (goto-char (+ (length cite-button-text) p))
+                (forward-line)
               ))))
       (move-to-column depth)
       (if (looking-at notmuch-show-signature-regexp)