[css-display] Make the run-in recursion a little clearer. Fixes #1640.

Author: tabatkins

css-display-3/Overview.bs

@@ -778,7 +778,7 @@ Run-In Layout</h2>
 			after its ''::marker'' pseudo-element's boxes (if any),
 			but preceding any other boxes generated by the contents of the block
 			(including the box generated by the ''::before'' pseudo-element, if any).
-			This step recurses if possible
+			This re-parenting recurses if possible
 			(so that the run-in effectively becomes
 			part of the deepest subsequent “paragraph” in its formatting context,
 			collecting newly-adjacent run-ins as it goes).
@@ -788,7 +788,8 @@ Run-In Layout</h2>
 			because property inheritance for non-anonymous boxes is based only on the element tree.</span>
 
 		<li>
-			Otherwise,
+			Otherwise
+			(if the <a>run-in sequence</a> is <em>not</em> followed by such a block),
 			an anonymous block box is generated around the <a>run-in sequence</a>
 			and all immediately following inline-level content
 			(up to, but not including, the next <a>run-in sequence</a>, if any).