Fix handling of persistent window parameters.

Fix handling of persistent window parameters.

* window.c (save_window_save, Fcurrent_window_configuration)
(Vwindow_persistent_parameters): Do not use Qstate.  Rewrite
doc-strings.
* window.el (window--state-get-1, window-state-get): Do not use
special state value for window-persistent-parameters.  Rename
argument IGNORE to WRITABLE.  Rewrite doc-string.
(window--state-put-2): Reset all window parameters to nil before
assigning values of persistent parameters.
* windows.texi (Window Configurations): Rewrite references to
persistent window parameters.
(Window Parameters): Fix description of persistent window
parameters.