Be explicit that Team rejections must be motivated

More details about typical motivations can go in /Guide

index.bs

@@ -1763,7 +1763,8 @@ Initiating Charter Refinement</h3>
 	An [=Advisory Committee representative=] <em class=rfc2119>may</em> formally request
 	that the [=Team=] initiate [=charter refinement=].
 	The Team <em class=rfc2119>may</em> deny such a request
-	if it thinks the proposal is insufficiently mature or does not align with W3C's scope and mission.
+	if it thinks the proposal is insufficiently mature or does not align with W3C's scope and mission,
+	and <em class=rfc2119>must</em> reply with its rationale.
 	This rejection is a [=Team Decision=],
 	and can be appealed only by 5 or more [=Members=],
 	through their [=Advisory Committee representative=],