[css-align] Per WG resolution, the default overflow alignment is 'true' for all layout modes.

Author: tabatkins

css-align/Overview.bs

@@ -398,9 +398,7 @@ Overflow Alignment: the ''safe'' and ''true'' keywords</h3>
 	while "safe" alignment changes the alignment mode in overflow situations in an attempt to avoid data loss.
 
 	If the <a>overflow alignment</a> isn't explicitly specified,
-	the default <a>overflow alignment</a> is determined by the layout mode.
-	Document-centric layout modes, such as block layout, default to "safe" <a>overflow alignment</a>,
-	while design-centric layout modes, such as flex layout, default to "true" <a>overflow alignment</a>.
+	the default <a>overflow alignment</a> is ''true''.
 
 	<pre class='prod'><dfn>&lt;overflow-position></dfn> = true | safe</pre>