[css-overflow] rename "mirror" to "both-edges"

See w3c#6349

Author: frivoal

css-overflow-4/Overview.bs

@@ -233,7 +233,7 @@ provided by the 'overflow' property.
 
 <pre class="propdef">
 Name: scrollbar-gutter
-Value: auto | stable && mirror?
+Value: auto | stable && both-edges?
 Initial: auto
 Applies to: [=scroll containers=]
 Inherited: no
@@ -306,7 +306,7 @@ 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>mirror</dfn>
+	<dt><dfn>both-edges</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.
@@ -1479,7 +1479,7 @@ but non-experimental implementation is not recommended.
 
 <pre class="propdef partial">
 Name: scrollbar-gutter
-New Values: auto | [ [ stable | always ] && mirror? && force? ] || match-parent
+New Values: auto | [ [ stable | always ] && both-edges? && force? ] || match-parent
 Applies to: all elements
 </pre>
 
@@ -1584,7 +1584,7 @@ The new values of this property have the following meaning:
 		Yet another possibility is to have a choice between growing the margin to protect the element,
 		or growing the padding to protect the element's content.
 
-		The syntax could be something like <code>scrollbar-avoid: none | [self | content] && mirror?</code>.
+		The syntax could be something like <code>scrollbar-avoid: none | [self | content] && both-edges?</code>.
 
 		An interesting consideration is that this may alleviate the need for ''scrollbar-gutter: match-parent'',
 		as it seems that situations that would have been addressed by ''scrollbar-gutter: stable'' or ''scrollbar-gutter: always'' on the parent
@@ -1681,7 +1681,7 @@ The new values of this property have the following meaning:
 
 	<dt><dfn>match-parent</dfn>
 	<dd>
-	On a [=block-level=] box whose parent has a [=scrollbar gutter=] (or mirrored gutters),
+	On a [=block-level=] box whose parent has a [=scrollbar gutter=] (or gutters on both edges),
 	this causes the box to have [=scrollbar gutter=]
 	on the same side(s) and of the same width as its parent’s gutter(s).
 	Moreover,