In window-related documentation write 'symbol' instead of 'constant'

In window-related documentation write 'symbol' instead of 'constant'

Suggested by Michael Heerdegen <michael_heerdegen@web.de>.

* src/window.c (window_restore_killed_buffer_windows): In
doc-string write 'symbol' instead of 'constant'.
* lisp/window.el (display-buffer--lru-window)
(display-buffer-use-least-recent-window): In doc-strings write
'symbol' instead of 'constant'.
* doc/lispref/windows.texi (Window Configurations): Write
'symbol' instead of 'constant'.

(cherry picked from commit 899ea79310d1b8ed78c3fd8ac1784043dd732dbf)