[css-inline-3] Add clarifying note about line boxes. #2546

Author: fantasai

css-inline-3/Overview.bs

@@ -45,6 +45,10 @@ Introduction</h2>
 	It inherits from its parent <a>block container</a>,
 	but is otherwise unstyleable.
 
+	Note: Line boxes, like <a>column boxes</a> [[css-multicol-1]],
+	are <a>fragmentation containers</a> generated by their <a>formatting context</a>
+	and are not part of the CSS <a>box tree</a>.
+
 <h2 id="line-height">
 Line Heights and Baseline Alignment</h2>