[css-values-4][editorial] Add note about 2p3 notation. Fixes #378

Author: tabatkins

css-values-4/Overview.bs

@@ -2174,6 +2174,10 @@ Absolute Lengths: the ''cm'', ''mm'', ''Q'', ''in'', ''pt'', ''pc'', ''px'' unit
 			<td>picas
 			<td>1pc = 1/6th of 1in
 
+				Note: Lengths in publishing contexts
+				are sometimes written like "2p3",
+				indicating a length of 2 picas and 3 points.
+				These can be written in CSS as ''calc(2pc + 3pt)''.
 
 		<tr><th><dfn id="pt">pt</dfn>
 			<td>points