[css-egg-1] Improved definition for ❓unit (to please @tabatkins)

Author: SebastianZ

css-egg-1/Overview.bs

@@ -452,7 +452,14 @@ this specification introduces three <dfn>random value units</dfn> covering a bro
 		<tr><th><dfn style="white-space: nowrap">❓</dfn>
 		    <td>Random value
 		    <td>Returns a <<number>>, <<dimension>>, or <<percentage>> between ''0'' and the given value,
-		        a <<color>>, <<string>>, or any kind of <a href="https://drafts.csswg.org/css-values-4/#keywords">pre-defined keyword</a>.
+		        a <<color-function>> with each of its color component values between ''0'' and the given value,
+		        a <<string>> with a number of characters between ''0'' and the given value rounded to the nearest integer,
+		        or any kind of <a href="https://drafts.csswg.org/css-values-4/#keywords">pre-defined keyword</a>
+		        starting with a [=letter index=] between ''0'' and the given value rounded to the nearest integer,
+				normalized to a [0, 25] range.
+
+				The <dfn>letter index</dfn> assigns numbers ''0'' through ''25'' to letters ''a'' through ''z'',
+				with ''0'' for ''a'' and ''25'' for ''z''.
 		<tr><th><dfn style="white-space: nowrap">🎁</dfn>
 		    <td>Complete randomness
 		    <td>May return any kind of value defined in [[CSS-VALUES-5]],