[css-text-4] Allow distribute to be a legacy value alias w3c#7322

For some engines this is easier; for some it's harder.
Spec allows either, since it's a compat alias and it doesn't matter.

Author: fantasai

css-text-4/Overview.bs

@@ -6919,6 +6919,7 @@ Justification Method: the 'text-justify' property</h3>
 			UAs must also support the alternate keyword <dfn>distribute</dfn>
 			which must compute to ''inter-character'',
 			thus having the exact same meaning and behavior.
+			UAs may treat this as a [=legacy value alias=].
 
 			<wpt>
 			text-justify/text-justify-003.html