Set up progressive vote threshold depending on participation

This mirrors the "requisite member vote" process from the W3C bylaws,
and avoids making momentous decisions based on a slim majority in vote
with low participation.

See w3c#886

index.bs

@@ -2806,9 +2806,18 @@ Advisory Committee Override</h3>
 	and “Abstain”,
 	together with Comments.
 
-	If the number of votes to approve
-	exceeds the number of votes to reject,
-	the proposal is approved.
+	The level of support needed for a [=Advisory Committee Override=] to pass
+	depends on the level of participation by [=Advisory Committee Representatives=],
+	(including those who cast an explicit “abstain” ballot):
+	* if 15% or fewer participate,
+		and the number of “Approve” ballots exceeds three times (3x) the number of “Reject” ballots,
+		the vote passes.
+	* if more than 15% but fewer than 20% participate,
+		and the number of “Approve” ballots exceeds twice (2x) the number of “Reject” ballots,
+		the vote passes.
+	* if 20% or more participate,
+		and the number of “Approve” ballots exceeds the number of “Reject” ballots,
+		the vote passes.
 
 	Otherwise, the proposal is rejected.