Improve `C-h b' output for remapped commands.

Improve `C-h b' output for remapped commands.

* lisp/help.el (describe-map): Include the bindings of the
remapped commands (bug#14084).

This will result in output like this, instead of just listing the
remapped commands:

<remap> <display-buffer>                ido-display-buffer
C-x 4 C-o                               display-buffer
<remap> <display-buffer-other-frame>    ido-display-buffer-other-frame
C-x 5 C-o                               display-buffer-other-frame
<remap> <insert-buffer>                 ido-insert-buffer
C-x x i                                 insert-buffer
<remap> <kill-buffer>                   ido-kill-buffer
C-x k                                   kill-buffer
<remap> <switch-to-buffer>              ido-switch-buffer
C-x b                                   switch-to-buffer
<remap> <switch-to-buffer-other-frame>  ido-switch-buffer-other-frame
C-x 5 b                                 switch-to-buffer-other-frame
<remap> <switch-to-buffer-other-window> ido-switch-buffer-other-window
C-x 4 b                                 switch-to-buffer-other-window