Add a note to icat --clear that it doesnt work with terminal multiplexers

Fixes #8839

Author: kovidgoyal

kittens/icat/main.py

@@ -42,7 +42,9 @@
 
 --clear
 type=bool-set
-Remove all images currently displayed on the screen.
+Remove all images currently displayed on the screen. Note that
+this cannot work with terminal multiplexers such as tmux since
+only the multiplexer can know the position of the screen.
 
 
 --transfer-mode