[css-backgrounds-3][editorial] Added change note for disallowing negative blur radii in 'box-shadow'

Author: SebastianZ

css-backgrounds-3/Overview.bs

@@ -3681,6 +3681,8 @@ Changes since the 14 February 2023 Candidate Recommendation Snapshot</h3>
 			[=CSS/value definition syntax=].
 		<li>Fixed the definition for where box shadows apply.
 			(<a href="https://github.com/w3c/csswg-drafts/issues/9286">Issue 9286</a>)
+		<li>Fixed syntax of 'box-shadow' to avoid allowing negative blur radii,
+			aligning it with the prose.
 	</ul>