[css-ui-4] Note that outline-offset is not part of the shorthand

See #8788

Author: frivoal

css-ui-4/Overview.bs

@@ -296,6 +296,11 @@ Outline properties</h2>
 	but without an explicit <<'outline-style'>> or <<'outline-color'>> value,
 	both 'outline-style' and 'outline-color' are set to ''outine-style/auto''.
 
+	Note: The shorthand purposefully omits the 'outline-offset' property,
+	which determines the position rather than the appearance of the outline,
+	so that it can cascade independently,
+	as well as for backwards compatibility reasons.
+
 	The outline created with the outline properties is drawn "over" a box,
 	i.e., the outline is always on top
 	and doesn't influence the position or size of the box,