Rewrite window-resizable.

Rewrite window-resizable.

* window.el (window-size-fixed-p): Rewrite doc-string.
(window-resizable-p): Rename to window--resizable-p.  Update
callers.
(window--resizable): New function.  Make all callers of
window-resizable call window--resizable instead.
(window-resizable): Rewrite in terms of window--resizable.
* windows.texi (Resizing Windows): Rewrite documentation of
window-resizable.