[css-box] Add missing <box> definition

Author: cdoublev

css-box-3/Overview.bs

@@ -211,7 +211,7 @@ The CSS Box Model {#box-model}
 Box-edge Keywords {#keywords}
 -----------------
 
-	The following CSS keywords are defined for use
+	The following <dfn>&lt;box></dfn> CSS keywords are defined for use
 	in properties (such as 'transform-box' and 'background-clip')
 	that need to refer to various box edges:
 

css-box-4/Overview.bs

@@ -197,7 +197,7 @@ The CSS Box Model {#box-model}
 Box-edge Keywords {#keywords}
 -----------------
 
-	The following CSS keywords are defined for use
+	The following <dfn>&lt;box></dfn> CSS keywords are defined for use
 	in properties (such as 'transform-box' and 'background-clip')
 	that need to refer to various box edges: