[css-rhythm] Add an issue and a clarification

Author: kojiishi

css-rhythm/Overview.bs

@@ -83,12 +83,15 @@ Adjusting Line Box Heights: the 'line-height-step' property {#line-height-step}
 	This adjustment is done
 	by assuming that there is an inline-level box that has adjusted A' and D'
 	in the line box.
+	This inline-level box does not affect alignment points of the 'vertical-align' property,
+	except values that align relative to the line box.
 
 	<div class="figure">
 	<img src="images/adjust-line-height.svg">
 	<p class="caption">Rounding up the computed line box height.
 	</div>
 
+	<div class="issue">'none' and '0' are equivalent. Is this ok?</div>
 	<div class="issue">Should this be animatable?
 	There doesn't seem to be use cases but needed for consistency?</div>