[css-transitions-2] Clarify about reversing parameters interaction with Web Animations API

Author: birtles

css-transitions-2/Overview.bs

@@ -129,6 +129,16 @@ on the {{CSSTransition}} object representing a [=running transition=]
 does <em>not</em> cause the transition to be disassociated
 from its [=owning element=].
 
+## Faster reversing of interrupted transitions ## {#reversing}
+
+The [=reversing shortening factor=] and [=reversing-adjusted start value=]
+associated with a transition in order to produce correct reverse behavior,
+are associated with the transition itself and not, for example,
+its [=animation effect=].
+As a result, transitions will use these same values when producing a
+reversed transition, even if the transition's associated [=animation effect=]
+has been updated or replaced using the Web Animations API [[WEB-ANIMATIONS]].
+
 ## The current transition generation ## {#current-transition-generation-section}
 
 Associated with each top-level browsing context is a <dfn>current transition