[css-display-3] Define “document order” since we use it in many places assuming it has a definition.

Author: fantasai

css-display-3/Overview.bs

@@ -1029,6 +1029,14 @@ Appendix A: Glossary</h2>
 
 			Note: some formatting contexts inhibit floating,
 			so that an element with ''float: left'' is not necessarily <a>out-of-flow</a>.
+
+		<dt><dfn>document order</dfn>
+		<dd>
+			The order in which boxes or content occurs in the document
+			(which can be different from the order in which it appears for rendering).
+			For the purpose of determining the relative order of pseudo-elements,
+			the box-tree order is used,
+			see [[css-pseudo-4#treelike]].
 	</dl>
 
 	See [[!CSS2]] <a href="https://www.w3.org/TR/CSS2/visuren.html">Chapter 9</a> for a fuller definition of these terms.