Clarify that nodes other than text and elements are treated as non-existent. Fixes w3c#4860.

Author: tabatkins

css-display-3/Overview.bs

@@ -57,6 +57,14 @@ Introduction</h2>
 	and <dfn id="text-nodes" oldids="css-text-node" export for=CSS lt="text node">text nodes</dfn>,
 	and renders it onto a <a href="https://www.w3.org/TR/CSS2/intro.html#canvas">canvas</a>
 	(such as your screen, a piece of paper, or an audio stream).
+	(Some source documents start from more complex trees,
+	such as the DOM, 
+	which can have comment nodes
+	and other types of things.
+	For the purposes of CSS,
+	all of these additional types of nodes are ignored,
+	as if they didn't exist.)
+
 	To do this, it generates an intermediary structure,
 	the <dfn id="box-tree" oldids="css-box-tree" export for=CSS>box tree</dfn>,
 	which represents the formatting structure of the rendered document.