[css-ruby] Introduce the term "interlinear annotation"

Author: frivoal

css-ruby-1/Overview.bs

@@ -1060,6 +1060,9 @@ Ruby Positioning: the 'ruby-position' property</h3>
 		</dd>
 	</dl>
 
+	[=Annotation containers=] that are not [=inter-character annotations=]
+	are called <dfn>interlinear annotations</dfn>.
+
 	If multiple [=ruby annotation containers=] have the same 'ruby-position',
 	they stack along the block axis,
 	with lower levels of annotation closer to the base text.