[css-values] Talk about bg-position

Author: tabatkins

css-values/Overview.bs

@@ -700,6 +700,10 @@ Percentages: the <<percentage>> type</h3>
 		not relative to any other value.
 		They cannot be combined with anything in ''calc()''
 		(except themselves, in useless expressions like ''calc(10% + 20%)'').
+
+		An exceptional case is 'background-position'--
+		there, <<percentage>>s and <<length>>s actually cause different behavior/interpretations.
+		However, there is prose defining how to interpret a ''calc()'' mixing the two in that context.
 	</div>
 
 <h3 id='dimensions'>