[css-grid-1] Note that absorbing positive free space means auto margins disable self-alignment.

Author: fantasai

css-grid-1/Overview.bs

@@ -3559,7 +3559,9 @@ Aligning with <a value for="margin">auto</a> margins</h3>
 
 		<li>
 			''margin/auto'' margins absorb positive free space
-			prior to alignment via the <a>box alignment properties</a>.
+			prior to alignment via the <a>box alignment properties</a>,
+			thereby disabling the effects of any [=self-alignment properties=]
+			in that axis.
 
 		<li>
 			Overflowing elements ignore their ''margin/auto'' margins