Clarified documentation of TweenChannel.arbitration_mode.

util/TweenChannel.gd

@@ -12,6 +12,13 @@ enum ArbitrationMode {
 }
 
 ## How to arbitrate between a tween request and an already running tween.
+##
+## Takes effect only for new calls to self.create_tween,
+## not in-progress calls. That is to say, if self.create_tween
+## has already been called at a time when arbitration_mode was YIELD,
+## and it changes while the call is yielding, then the in-progress call
+## still will not return until its turn, as though arbitration_mode
+## were still YIELD.
 var arbitration_mode := ArbitrationMode.YIELD
 ## Tween currently running if any, else null.
 var current_tween: Tween = null