patch 8.1.1321: no docs or tests for listener functions
Problem: No docs or tests for listener functions.
Solution: Add help and tests for listener_add() and listener_remove().
Invoke the callbacks before redrawing.
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index c5317e2..866664c 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -812,6 +812,8 @@
setbufline() replace a line in the specified buffer
appendbufline() append a list of lines in the specified buffer
deletebufline() delete lines from a specified buffer
+ listener_add() add a callback to listen to changes
+ listener_remove() remove a listener callback
win_findbuf() find windows containing a buffer
win_getid() get window ID of a window
win_gotoid() go to window with ID