[css-align-3] Clarify that minimum spacing is in addition to, not in place of, margins. w3c#2336

Author: fantasai

css-align-3/Overview.bs

@@ -1865,9 +1865,10 @@ Row and Column Gutters: the 'row-gap' and 'column-gap' properties</h3>
 		<dd>
 			When applied to the <a>main axis</a>
 			(e.g. 'column-gap' in a ''flex-flow/row'' <a>flex container</a>),
-			indicates minimum spacing between items,
-			as if a fixed-size margin were inserted between adjacent <a>flex items</a>
-			in a single line.
+			indicates minimum spacing between items
+			(as if an additional fixed-size margin were inserted
+			between adjacent <a>flex items</a>
+			in a single line).
 
 			When applied to the <a>cross axis</a>
 			(e.g. 'row-gap' in a ''flex-flow/row'' <a>flex container</a>),