[css-align-3] Clarify conjunction for list of conditions

Author: fantasai

css-align-3/Overview.bs

@@ -2171,7 +2171,7 @@ Baseline Alignment Grouping</h3>
 
 	A <dfn export>baseline-sharing group</dfn>
 	is composed of boxes that participate in baseline alignment together.
-	This is possible only if they
+	This is possible only if they both:
 
 	<ul>
 		<li>Share an <a>alignment context</a> along an axis perpendicular to the axis they're being baseline-aligned in.