Second attempt to fix some doc-strings in window.c.

* window.c (Fwindow_new_total, Fwindow_new_normal)
(Fwindow_new_pixel, Fset_window_new_pixel)
(Fset_window_new_total, Fset_window_new_normal): Second attempt
to fix the doc-strings of these functions.  See:
http://lists.gnu.org/archive/html/bug-gnu-emacs/2014-08/msg00287.html
In display-buffer-below-selected reuse window below selected if it shows buffer already (Bug#18181).

* window.el (display-buffer-below-selected): Restore original
behavior if buffer is already displayed in the window below the
selected one (Bug#18181).