[css-values-4] Allow {A} and ? multipliers to be stacked (w3c#12521)

The Value Definition Syntax defines the list of multipliers that can be
stacked.

The syntax definition of `<cursor-image>` in css-ui-4 has started to use
`<number>{2}?` but the `{A}?` stack wasn't explicitly allowed (and some CSS
parsers do not support it yet).

This update completes the list of multipliers that can be stacked with `{A}?`
and `{A,B}?`.

Author: tidoust

css-values-4/Overview.bs

@@ -335,7 +335,7 @@ Component Value Multipliers</h3>
 	</ul>
 
 	The <css>+</css> and <css>#</css> multipliers may be stacked as ''+#'';
-	similarly, the <css>#</css> and <css>?</css> multipliers may be stacked as ''#?''.
+	similarly, the <css>#</css> and <css>?</css> multipliers, <css>{A}</css> and <css>?</css> multipliers, and <css>{A,B}</css> and <css>?</css> multipliers may be stacked as ''#?'', ''{A}?'', and ''{A,B}?'', respectively.
 	These stacks each represent the later multiplier
 	applied to the result of the earlier multiplier.
 	(These same stacks can be represented using grouping,