Allow 'display-buffer' to set up body size of chosen window (Bug#17065)

Allow 'display-buffer' to set up body size of chosen window (Bug#17065)

* doc/lispref/windows.texi (Buffer Display Action Alists):
Describe new possible values for 'window-height', 'window-width'
and 'window-size' action alist entries.
* lisp/window.el (window--display-buffer): Handle new values for
'window-height', 'window-width' and 'window-size' ALIST entries.
(display-buffer): Update doc-string for new values of some
action alist entries.
* etc/NEWS: Mention new values of some action alist entries.