[css-overflow-3] clarify the allowance for inserting the block ellipsis visually

See w3c#2905

Author: frivoal

css-overflow-3/Overview.bs

@@ -875,9 +875,15 @@ Indicating Block-Axis Overflow: the 'block-ellipsis' property</h3>
 
 	If there is no next <a>fragmentation container</a>
 	and thus the remainder of the content after the break would be discarded,
-	then the UA may visually replace the contents of the line,
+	then the UA may instead visually replace the contents of the line,
 	as it does for 'text-overflow'.
 
+	Note: Allowing the UA to visually replace the content of the line as with 'text-overflow'
+	is allowed as an implementation convenience,
+	to avoid blocking the whole feature on a comparatively small detail.
+	However, that behavior is less desirable,
+	and may be removed in later levels of this specification.
+
 	If there is a subsequent <a>fragmentation container</a> in the [=fragmentation context=]
 	that would receive subsequent content,
 	then the content displaced by the <a>block overflow ellipsis</a>