[css-grid] Mention gutters up front, since authors care a lot even though its a fairly minor detail in the layout model.

Author: fantasai

css-grid/Overview.bs

@@ -439,6 +439,8 @@ Grid Tracks and Cells</h3>
 	Each <a>grid track</a> is assigned a sizing function,
 	which controls how wide or tall the column or row may grow,
 	and thus how far apart its bounding <a>grid lines</a> are.
+	Subsequent <a>grid tracks</a> can be separated by <a href="#gutters">gutters</a> or <a href="#grid-align">alignment spacing</a>,
+	but are otherwise immediately adjacent.
 
 	A <dfn export>grid cell</dfn> is the similar term for the full grid&mdash;it
 	is the space between two adjacent row and two adjacent column <a>grid lines</a>.