The docstring of ido-find-alternate-file is wrong:

;;;###autoload
(defun ido-find-file-other-window ()
  "Switch to another file and show it in another window.
The file name is selected interactively by typing a substring.
For details of keybindings, see `ido-find-file'."
  (interactive)
  (ido-file-internal 'other-window 'find-file-other-window))

;;;###autoload
(defun ido-find-alternate-file ()
  "Switch to another file and show it in another window.
The file name is selected interactively by typing a substring.
For details of keybindings, see `ido-find-file'."
  (interactive)
  (ido-file-internal 'alt-file 'find-alternate-file nil "Find alternate file: "))

The first line of ido-find-alternate-file could be changed to "Find file FILENAME, select its buffer, kill previous buffer." to fix this.

Cheers,
Clément.