[css-ruby] M/B/P do not apply to base/annotation containers

Closes w3c#4937

Author: frivoal

css-ruby-1/Overview.bs

@@ -710,9 +710,16 @@ Inter-character Ruby Layout</h3>
 <h3 id="box-style">
 Styling Ruby Boxes</h3>
 
-	In most respects, ruby boxes can be styled similar to inline boxes.
-	However, the UA is not required to support
-	any of the box properties (borders, margins, padding),
+	The 'margin', 'border', and 'padding' properties
+	do not apply to [=ruby base container boxes=]
+	nor to [=ruby annotation container boxes=].
+
+	Issue: Should they also not apply to
+	<a href="#nested-pairing">ruby-internal ruby container boxes</a>?
+	If they do, what does that mean?
+
+	Additionally,
+	the UA is not required to support
 	any of the background properties or outline properties,
 	or any other property that illustrates the bounds of the box
 	on [=ruby base container boxes=], [=ruby annotation container boxes=],