[css-cascade] Add subheading for cascade order

Author: fantasai

css-cascade-3/Overview.bs

@@ -649,6 +649,9 @@ Cascading</h2>
 	sorts them by their declaration’s precedence as determined below,
 	and outputs a single <a>cascaded value</a>.
 
+<h3 id="cascade-order">
+Cascade Order</h3>
+
 	The cascade sorts declarations according to the following criteria,
 	in descending order of priority:
 

css-cascade-4/Overview.bs

@@ -763,6 +763,9 @@ Cascading</h2>
 	sorts them by their declaration’s precedence as determined below,
 	and outputs a single <a>cascaded value</a>.
 
+<h3 id="cascade-order">
+Cascade Order</h3>
+
 	The cascade sorts declarations according to the following criteria,
 	in descending order of priority: