[css-grid-1] Add note reminding implementers that gutters are treated…

… like fixed-size tracks, and need to be incorporated into track size calculations accordingly. #2201

css-grid-1/Overview.bs

@@ -3617,6 +3617,12 @@ Track Sizing Terminology</h3>
 			The number of <a>grid tracks</a> crossed by a <a>grid item</a> in the applicable dimension.
 	</dl>
 
+	Note: Remember that <a href="#gutters">gutters</a> are treated as fixed-size tracks--
+	tracks with their min and max sizing functions both set to the gutter’s used size--
+	for the purpose of the grid sizing algorithm.
+	Their widths need to be incorporated into
+	the <a>track sizing algorithm</a>’s calculations accordingly.
+
 <h3 id="algo-track-sizing">
 Track Sizing Algorithm</h3>