[css-values-5] Make it clear that allow-keywords can't just interpolate with intrinsic sizes. (#11179)

Author: Psychpsyo

css-values-5/Overview.bs

@@ -2696,7 +2696,7 @@ Interpolating sizing keywords: the 'interpolate-size' property</h3>
 
 		<dt><dfn>allow-keywords</dfn>
 		<dd>
-			Two values can be interpolated if
+			Two values can also be interpolated if
 			one of them is an <<intrinsic-size-keyword>>
 			and the other is a <<length-percentage>>.
 			This is done by treating