[css-box-4] Clarify that only descendant margins are trimmed by margin-trim. w3c#8283

Author: fantasai

css-box-4/Overview.bs

@@ -389,7 +389,8 @@ Margins at Container Edges: the 'margin-trim' property {#margin-trim}
 			For in-flow boxes and floats contained by this box,
 			margins adjacent to the box’s specified edges
 			are truncated to zero.
-			It also truncates any margins collapsed with such a margin.
+			It also truncates any descendant margins collapsed with such a margin
+			(but not its own, its siblings’, or its ancestors’).
 
 		<dt><dfn>block</dfn>
 		<dd>
@@ -588,6 +589,13 @@ Borders {#borders}
 	Both sets of properties control the same set of borders:
 	they are just different ways of indexing each side.
 
+Recent Changes {#changes}
+==============
+
+	Changes since the previous Working Draft include:
+	* Clarifying that only descendant margins are trimmed by 'margin-trim'
+		even if self or sibling margins would have collapsed with the trimmed margin.
+
 Changes Since CSS Level 3 {#changes-since-3}
 =========================