clarify shape-margin and float area interaction

Author: astearns

css-shapes-1/Overview.bs

@@ -116,7 +116,8 @@ Terminology</h3>
 	the <a>float area</a> 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 <a>float area</a>.
 
@@ -1035,6 +1036,12 @@ Expanding a Shape: the 'shape-margin' property</h3>
 	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 <a>float area</a>, 
+	and must be constructed  
+	before making any <a>wrap</a> decisions.
+
 	This property takes only non-negative values.
 
 	<pre class='propdef'>
@@ -1168,6 +1175,7 @@ Change Log</h2>
 	<ul>
 		<li>Added vertex rounding for polygon() for <a href="https://github.com/w3c/csswg-drafts/issues/9843">issue #9843</a> </li>
 		<li>Updated shape-outside prose to include all <<basic-shape>>s for <a href="https://github.com/w3c/csswg-drafts/issues/9728">issue #9728</a> </li>
+		<li>Clarified the shape-margin contribution to float areas for <a href="https://github.com/w3c/csswg-drafts/issues/2949">issue #2949</a> </li>
 	</ul>
 
 <h3 class="no-num" id="20140320">