doc clarified

core/src/main/java/hudson/model/RunAction.java

@@ -31,6 +31,9 @@ package hudson.model;
 public interface RunAction extends Action {
     /**
      * Called after the build is loaded and the object is added to the build list.
+     * 
+     * Because {@link RunAction}s are persisted with {@link Run}, the implementation
+     * can keep a reference to {@link Run} in a field (which is set via {@link #onAttached(Run)})
      */
     void onLoad();