[css-rhythm] Minor editorial from a feedback

Author: kojiishi

css-rhythm/Overview.bs

@@ -122,7 +122,15 @@ Adjusting Line Box Heights: the 'line-height-step' property {#line-height-step}
 	Computed Value: the absolute length, 0 for none
 	</pre>
 
-	This property defines the <dfn>step unit</dfn> for line box heights.
+This property defines the <dfn>step unit</dfn> for line box heights.
+A <a>step unit</a> is the length of the vertical rhythm,
+usually the distance from one baseline to the next baseline of the body text.
+Body text should fit into one <a>step unit</a>,
+and taller lines such as headings
+should have heights of two or more <a>step unit</a>s.
+Vertical rhythm is created
+by making heights of all lines an integer multiple of the <a>step unit</a>.
+
 	When the <a>step unit</a> is set to a positive <<length>>,
 	the line box heights are rounded <i>up</i> to
 	the closest multiple of the unit.