clarify shape-margin and float area interaction

Author: astearns

css-shapes-1/Overview.bs

@@ -116,7 +116,8 @@ Terminology
 	the float area produced
 	by the ''margin-box'' value,
 	which includes border-radius curvature).
-	This specification's 'shape-outside' property
+	This specification's 'shape-outside' 
+	and 'shape-margin' properties
 	can be used to define an arbitrary,
 	non-rectangular float area.
 
@@ -1035,6 +1036,12 @@ Expanding a Shape: the 'shape-margin' property
 	take all points
 	on the circle centered at the point
 	and with a radius of 'shape-margin'.
+
+	The new shape produced by applying 'shape-outside' 
+	is what determines the float area, 
+	and must be constructed  
+	before making any wrap decisions.
+
 	This property takes only non-negative values.
 
 	
@@ -1168,6 +1175,7 @@ Change Log
 	
 		Added vertex rounding for polygon() for issue #9843 
 		Updated shape-outside prose to include all <>s for issue #9728 
+		Clarified the shape-margin contribution to float areas for issue #2949