[css-ruby] Distinguish stacking direction: intra-character vs intralinear

Author: frivoal

css-ruby-1/Overview.bs

@@ -1064,7 +1064,8 @@ Ruby Positioning: the 'ruby-position' property</h3>
 	are called <dfn>interlinear annotations</dfn>.
 
 	If multiple [=ruby annotation containers=] have the same 'ruby-position',
-	they stack along the block axis,
+	they stack along the block axis for [=interlinear annotations=],
+	and along the inline axis for [=inter-character annotations=],
 	with lower levels of annotation closer to the base text.
 
 <h3 id="collapsed-ruby">