[css-syntax] Add another an+b difference from 2.1.

Author: tabatkins

css-syntax/Overview.html

@@ -3709,6 +3709,9 @@ <h3 id=changes-from-css-2.1-core-grammar><span class=secno>5.5. </span>
       "+" and "n". (This is an accidental consequence of the "+" and "n"
       parsing as separate CSS tokens, and CSS's value grammar ignoring
       whitespace.)
+
+     <li> In some cases, "-" characters or digits can be escaped (when they
+      appear as part of the unit of a 〈dimension〉 or 〈ident〉).
     </ul>
   </ol>
 

css-syntax/Overview.src.html

@@ -2748,6 +2748,10 @@ <h3>
 					a space is now allowed between the "+" and "n".
 					(This is an accidental consequence of the "+" and "n" parsing as separate CSS tokens,
 					and CSS's value grammar ignoring whitespace.)
+
+				<li>
+					In some cases, "-" characters or digits can be escaped
+					(when they appear as part of the unit of a 〈dimension〉 or 〈ident〉).
 			</ul>
 	</ol>