Fix and rewrite doc on window recombining facilities.

Fix and rewrite doc on window recombining facilities.

* window.c (Fsplit_window_internal): Set combination limit of
new parent window to t iff Vwindow_combination_limit is t;
fixing a regression introduced with the change from 2012-09-22.
(Fwindow_combination_limit, Fset_window_combination_limit): Fix
doc-strings.

* elisp.texi (Top): Add Recombining Windows to menu.
* windows.texi (Recombining Windows): New subsection.
(Splitting Windows): Rewrite text on handling of window
combinations and move it to new subsection.