[css-overflow-4] rename ''both'' to ''mirror''

See #6349

css-overflow-4/Overview.bs

@@ -233,7 +233,7 @@ provided by the 'overflow' property.
 
 <pre class="propdef">
 Name: scrollbar-gutter
-Value: auto | stable && both?
+Value: auto | stable && mirror?
 Initial: auto
 Applies to: [=scroll containers=]
 Inherited: no
@@ -306,17 +306,10 @@ The values of this property have the following meaning:
 	Note: This does not change whether the scrollbar itself is visible,
 	only the presence of a gutter is affected.
 
-	<dt><dfn>both</dfn>
+	<dt><dfn>mirror</dfn>
 	<dd>If a <a>scrollbar gutter</a> would be present
 	on one of the inline start edge or the inline end edge of the box,
 	another <a>scrollbar gutter</a> must be present on the opposite edge as well.
-
-	Issue: ''both'' refers to both-sides,
-	but that may not be obvious to authors without reference to the specification or documentation.
-	<code>both-sides</code> would be clearer, but longer.
-	<code>symmetrical</code> is also clearer, but also longer and harder to spell.
-	While still not as short,
-	<code>mirror</code> might be a good compromise between length and understandability.
 </dl>
 
 When the <a>scrollbar gutter</a> is present but the scrollbar is not,
@@ -1479,7 +1472,7 @@ but non-experimental implementation is not recommended.
 
 <pre class="propdef partial">
 Name: scrollbar-gutter
-New Values: auto | [ [ stable | always ] && both? && force? ] || match-parent
+New Values: auto | [ [ stable | always ] && mirror? && force? ] || match-parent
 Applies to: all elements
 </pre>