[css-inline] Clarify interaction of inline-sizing and ruby

This avoids circularities.

Author: frivoal

css-inline-3/Overview.bs

@@ -1380,7 +1380,7 @@ Inline Box Heights: the 'inline-sizing' property</h3>
 	Name: inline-sizing
 	Value: normal | stretch
 	Initial: normal
-	Applies to: <a>inline boxes</a>
+	Applies to: <a>inline boxes</a>, but not [=ruby container boxes=] nor [=internal ruby boxes=]
 	Inherited: no
 	Percentages: n/a
 	Computed value: specified keyword