Added a note to progressbar documentation (#5781)

Fixes #5211

Thanks to @pigati23 and everyone else who was involved.

doc/script_commands.txt

@@ -6899,6 +6899,10 @@ character moves while the progress bar progresses, it is aborted and
 the script ends. The color format is in RGB (RRGGBB). The color is
 currently ignored by the client and appears always green.
 
+NOTE:
+Ragexe clients are known to randomly crash if a message window is still open.
+If possible make sure to close all message windows before triggering the progressbar command.
+
 ---------------------------------------
 
 *progressbar_npc "<color>",<seconds>{,<"NPC Name">};