[css-flexbox] Define the automatic block size of a flex container, since it's technically not defined by CSS2. w3c#4905

Author: tabatkins

css-flexbox-1/Overview.bs

@@ -2612,6 +2612,13 @@ Line Length Determination</h3>
 		<li id='algo-main-container'>
 			<strong>Determine the <a>main size</a> of the flex container</strong>
 			using the rules of the formatting context in which it participates.
+
+			<div class=note>
+				The [=automatic block size=] of a [=block-level=] [=flex container=]
+				is its [=max-content size=].
+
+				The Block Layout spec should define this equivalency, but it doesn't exist yet.
+			</div>
 	</ol>
 
 <h3 id='main-sizing'>