[css-ruby-1] Clarify 'ruby-position: separate'.

Author: fantasai

css-ruby-1/Overview.bs

@@ -1347,7 +1347,9 @@ Sharing Annotation Space: the 'ruby-merge' property</h3>
 	<dl dfn-for=ruby-merge dfn-type=value>
 		<dt><dfn>separate</dfn>
 		<dd>
-			Each ruby annotation box is rendered in the same [=column=](s) as its corresponding base box(es).
+			Each ruby annotation box is rendered within
+			the same [=column=](s) as its corresponding base box(es),
+			i.e. without overlapping adjacent bases on either side.
 			This style is called “mono ruby” in [[JLREQ]].
 
 			<div class="example">