[css-display] Normatively define what was in the previous note: that run-in fixup occurs before first line determination.

Author: fantasai

css-display/Overview.bs

@@ -780,7 +780,11 @@ Run-In Layout</h2>
 	or to have out-of-flow boxes impede the running-in of earlier boxes.
 	Implementers and authors are encouraged to contact the CSSWG if they have a preferred behavior, as this one was picked somewhat at random.
 
-	This fixup occurs before the anonymous block and inline box fixup described in <a href="https://www.w3.org/TR/CSS21/visuren.html#box-gen">CSS2&sect;9.2</a>.
+	This fixup occurs before the anonymous block and inline box fixup described in <a href="https://www.w3.org/TR/CSS21/visuren.html#box-gen">CSS2&sect;9.2</a>,
+	and affects the determination of the <a>first formatted line</a>
+	of the affected elements
+	as if the <a>run-in sequence</a> were originally
+	in its final location in the box tree.
 
 	Note: As the earliest run-in represents the first text on the <a>first formatted line</a> of its containing block,
 	a ''::first-letter'' pseudo-element applied to that block element