[css-counter-styles] Define speak-as:auto for system:override styles. Eliminate 'computed to' wording, as there's no computed values in at-rules.

--HG--
extra : rebase_source : 6448f40d8b88b259ea14563c6438272c4a83e554

Author: tabatkins

css-counter-styles/Overview.bs

@@ -877,9 +877,13 @@ Speech Synthesis: the 'speak-as' descriptor</h3>
 	<dl dfn-for="@counter-style/speak-as" dfn-type=value>
 		<dt><dfn>auto</dfn>
 		<dd>
-			If the counter style's 'system' is <a value for=system>alphabetic</a>, this value computes to <a value for=speak-as>alphabetic</a>.
-			If the 'system' is ''cyclic'', this value computes to ''bullet''.
-			Otherwise, this value computes to <a value for=speak-as>numeric</a>.
+			If the counter style's 'system' is ''system/alphabetic'',
+			this value has the same effect as ''speak-as/alphabetic''.
+			If the 'system' is ''cyclic'',
+			this value has the same effect as ''bullet''.
+			If the 'system' is ''override'',
+			this value has the same effect as the overridden style's 'speak-as'.
+			Otherwise, this value has the same effect as ''speak-as/numeric''.
 
 		<dt><dfn>numeric</dfn>
 		<dd>