[css-grid] Add note about gutters being treated as fixed-size tracks.

Author: tabatkins

css-grid/Overview.bs

@@ -3179,6 +3179,8 @@ Initialize Track Sizes</h3>
 	In all cases, if the <a>growth limit</a> is less than the <a>base size</a>,
 	increase the <a>growth limit</a> to match the <a>base size</a>.
 
+	Note: <a>Gutters</a> are treated as fixed-size tracks for the purpose of the <a>track sizing algorithm</a>.
+
 <h3 id="algo-content">
 Resolve Intrinsic Track Sizes</h3>