[css-box-4] Gloss in-flow jargon (w3c#11521)

Author: cvrebert

css-box-4/Overview.bs

@@ -402,15 +402,15 @@ Margins at Container Edges: the 'margin-trim' property {#margin-trim}
 		<dt><dfn>block-start</dfn>
 		<dt><dfn>block-end</dfn>
 		<dd>
-			For in-flow boxes contained by this box,
+			For [=in-flow=] boxes contained by this box,
 			margins adjacent to the box’s specified edges
 			are truncated to zero.
 			It also truncates any descendant margins collapsed with such a margin
 			(but not its own, its siblings’, or its ancestors’).
 
 		<dt><dfn>inline-start</dfn>
 		<dt><dfn>inline-end</dfn>
-			For in-flow boxes contained by this box,
+			For [=in-flow=] boxes contained by this box,
 			margins adjacent to the box’s specified edges
 			are truncated to zero.
 			(However, these values do not apply to [=block containers=])