[css-page-3][editorial] Add "auto" to page-margin-safety value definition

Author: cdoublev

css-page-3/Overview.bs

@@ -2072,7 +2072,7 @@ Staying within the printable area: the '@page/page-margin-safety' property</h3>
 	<pre class='descdef'>
 	Name: page-margin-safety
 	For: @page
-	Value: none | clamp | add
+	Value: none | auto | clamp | add
 	Initial: auto
 	Computed Value: as specified
 	</pre>