Add FIXMEs to def-setting!'s

They will be removed on the 2.1.1 release.

modules/app/email/autoload/email.el

@@ -40,6 +40,7 @@ default/fallback account."
         (setq-default mu4e-context-current context))
       context)))
 
+;; FIXME obsolete :email
 ;;;###autoload
 (def-setting! :email (label letvars &optional default-p)
   :obsolete set-email-account!