=tck reactive-streams#138 clarify docs on registerOnSubscribe

Aims to clarify confusion about this method, as seen in reactive-streams#138

Author: ktoso

tck/src/main/java/org/reactivestreams/tck/SubscriberWhiteboxVerification.java

@@ -749,7 +749,8 @@ public void registerOnSubscribe(SubscriberPuppet p) {
   public interface SubscriberPuppeteer {
 
     /**
-     * Must be called by the test subscriber when it has received the `onSubscribe` event.
+     * Must be called by the test subscriber when it has successfully registered a subscription
+     * inside the `onSubscribe` method.
      */
     void registerOnSubscribe(SubscriberPuppet puppet);
   }
@@ -782,4 +783,4 @@ public interface SubscriberPuppet {
   public void notVerified() {
     throw new SkipException("Not verified using this TCK.");
   }
-}
+}