[css-grid] Add note requested in <https://lists.w3.org/Archives/Public/www-style/2016Jan/0145.html>

Author: fantasai

css-grid/Overview.bs

@@ -3276,6 +3276,8 @@ Gutters: the 'grid-column-gap', 'grid-row-gap', and 'grid-gap' properties</h3>
 
 	If a <a>grid</a> is <a>fragmented</a> between tracks,
 	the <a>gutter</a> spacing between those tracks must be suppressed.
+	<span class="note">Note that gutters are suppressed even after forced breaks,
+	<a href="https://www.w3.org/TR/css-break-3/#break-margins">unlike margins</a>.</span>
 
 	<pre class='propdef'>
 	Name: grid-gap