Rename window-splits to window-combination-resize.

Rename window-splits to window-combination-resize.

* window.c (Vwindow_splits): Rename to
Vwindow_combination_resize.  Suggested by Juri Linkov.
(Fsplit_window_internal): Use Vwindow_combination_resize instead
of Vwindow_splits.
* window.el (window-resize, delete-window, split-window):
Replace window-splits by window-combination-resize.
* cus-start.el (window-splits): Replace by
window-combination-resize.
* windows.texi (Resizing Windows, Splitting Windows)
(Deleting Windows): Use term window-combination-resize instead
of window-splits.