[css-transitions-2] Move transition generation to separate section

Author: birtles

css-transitions-2/Overview.bs

@@ -81,6 +81,8 @@ the reversing shortening factor.
 
 # Starting of transitions # {#starting}
 
+## The current transition generation ## {#current-transition-generation-section}
+
 Associated with each top-level browsing context is a <dfn>current transition
 generation</dfn> that is incremented on each <a>style change event</a>.