[css-values-4][editorial] Add a note explaining why position serializ…

…ation violates the shortest-serialization-principle. #8996

css-values-4/Overview.bs

@@ -2857,6 +2857,12 @@ Serializing <<position>></h4>
 			* Components are serialized horizontal first, then vertical.
 	</dl>
 
+	Note: <<position>> values are never serialized as a single value,
+	even when a single value would produce the same behavior,
+	to avoid causing parsing ambiguities in some grammars
+	where a <<position>> is placed next to a <<length>>,
+	such as 'transform-origin'.
+
 	Note: [=Computed values=] are always serialized as two offsets (without keywords)
 	because the [=computed value=] does not preserve syntactic distinctions.