[css-display-3] Define block containers that establish new BFCs to have a *used* display value of 'flow-root'. This is purely editorial. Resolves #1550.

Author: fantasai

css-display-3/Overview.bs

@@ -387,6 +387,8 @@ Inner Display Layout Models: the ''flow'', ''flow-root'', ''table'', ''flex'', '
 			it either establishes a new <a>block formatting context</a> for its contents
 			or integrates its contents into its parent <a>formatting context</a>.
 			See <a href="https://www.w3.org/TR/CSS2/visuren.html">CSS2.1 Chapter 9</a>. [[!CSS2]]
+			A <a>block container</a> which establishes a new <a>block formatting context</a>
+			is considered to have a used 'display' value of ''flow-root''.
 
 		<dt><dfn>flow-root</dfn>
 		<dd>