[css-position-4] Fix the 'only render first box' to make more sense, and document where it came from.

Author: tabatkins

css-position-4/Overview.bs

@@ -382,10 +382,11 @@ are rendered with the following qualities:
 
 * Its parent stacking context is the root stacking context.
 
-* If it consists of multiple layout boxes, the first box is used.
+* If the element generates multiple boxes due to fragmentation,
+	only the first such box is rendered;
+	additional boxes are discarded without rendering.
 
-	Issue: Not sure what this means.
-	Maybe it's talking about fragmentation?
+	<!-- https://www.w3.org/Bugs/Public/show_bug.cgi?id=24523 -->
 
 * It is rendered as an atomic unit as if it were a sibling of the document's root.