[css-rhythm] Add 'none' value

Requested in issue w3c#938

Author: kojiishi

css-rhythm/Overview.bs

@@ -57,14 +57,14 @@ Adjusting Line Box Heights: the 'line-height-step' property {#line-height-step}
 
 	<pre class='propdef'>
 	Name: line-height-step
-	Value: <<length>>
-	Initial: 0
+	Value: none | <<length>>
+	Initial: none
 	Applies to: block containers
 	Inherited: yes
 	Animatable: no
 	Percentages: N/A
 	Media: visual
-	Computed Value: the absolute length
+	Computed Value: the absolute length for length
 	</pre>
 
 	This property defines the <dfn>step unit</dfn> for line box heights.