bug#9293: say what they are shadowed by

[jidanni]

   Tar mode defined in `tar-mode.el':...
   0 .. 9               digit-argument  (binding currently shadowed)

Shadowed by what?

   e .. f               tar-extract  (binding currently shadowed)

   C-d          tar-flag-deleted
   RET          tar-extract
     (that binding is currently shadowed by another mode)
   C-n          tar-next-line
   C-p          tar-previous-line
   SPC          tar-next-line
     (that binding is currently shadowed by another mode)

Say what mode on each of such lines!