patch 9.1.0500: cannot switch buffer in a popup

Problem:  cannot switch buffer in a popup
          (Yggdroot)
Solution: add popup_setbuf() function

fixes: #15006
closes: #15026

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 60d0d1e..a54a991 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -1319,6 +1319,7 @@
 	popup_move()		change the position and size of a popup
 	popup_setoptions()	override options of a popup
 	popup_settext()		replace the popup buffer contents
+	popup_setbuf()		set the popup buffer
 	popup_close()		close one popup
 	popup_clear()		close all popups
 	popup_filter_menu()	select from a list of items