Document values of window-system and deprecate its use as predicate.

Document values of window-system and deprecate its use as predicate.

 src/frame.c (Fframep, Fwindow_system): Deprecate use as a predicate.
 Document all values.
 src/dispnew.c (syms_of_display) <initial-window-system, window-system>:
 Deprecate use as a boolean flag.  Document all values.
 src/display.texi (Window Systems): Deprecate use of window-system as
 a predicate.