read-multiple-choice: Improve key formatting

* lisp/emacs-lisp/rmc.el (rmc--add-key-description): Improve
formatting of key missing in the description.  Use face help-key-name
on terminals that can't display underline.

lisp/emacs-lisp/rmc.el

@@ -38,10 +38,10 @@
            ;; Not in the name string, or a special character.
            ((or (not pos)
                 (member desc '("ESC" "TAB" "RET" "DEL" "SPC")))
-            (format "[%s] %s"
+            (format "%s %s"
                     (if graphical-terminal
                         (propertize desc 'face 'read-multiple-choice-face)
-                      desc)
+                      (propertize desc 'face 'help-key-name))
                     name))
            ;; The prompt character is in the name, so highlight
            ;; it on graphical terminals.