Improve documentation of 'display-buffer-*' functions

Improve documentation of 'display-buffer-*' functions

* lisp/window.el (display-buffer-in-atom-window)
(display-buffer-in-side-window, display-buffer-same-window)
(display-buffer--maybe-same-window)
(display-buffer-reuse-window)
(display-buffer-reuse-mode-window)
(display-buffer-pop-up-frame, display-buffer-pop-up-window)
(display-buffer--maybe-pop-up-frame-or-window)
(display-buffer--maybe-pop-up-frame)
(display-buffer-in-child-frame, display-buffer-in-direction)
(display-buffer-below-selected, display-buffer-at-bottom)
(display-buffer-in-previous-window)
(display-buffer-use-some-window, display-buffer-no-window):
More details about the ALIST argument in the doc string.  (Bug#19461)