Replace display-buffer-window by help-setup window parameter.

Replace display-buffer-window by help-setup window parameter.

* window.el (display-buffer-window): Remove.
(display-buffer-record-window): Use help-setup window parameter
instead of variable display-buffer-window.
(display-buffer-function, special-display-buffer-names)
(special-display-function): Mention help-setup parameter instead
of display-buffer-window in doc-string.
* help.el (help-window-setup): New argument help-window.  Use
help-window-setup parameter instead of display-buffer-window.
Reword some messages.
(with-help-window): Pass window used for displaying the buffer
to help-window-setup.  Don't set display-buffer-window.