(ido-mode-map): Doc fix.

(ido-mode-map): Doc fix.
(ido-mode-common-map, ido-mode-file-map)
(ido-mode-file-dir-map, ido-mode-buffer-map): New keymaps.
(ido-define-mode-map): Rewrite.  Select one of the new maps as
parent for ido-mode-map instead of building from scratch.
(ido-init-mode-maps): New defun to initialize new maps.
(ido-mode): Call it.
(ido-switch-buffer): Doc fix -- use \<ido-mode-buffer-map>.
(ido-find-file): Doc fix -- use \<ido-mode-file-map>.