Add a small comment to a test

Addresses comments on aosp/1771187

Test: comment-only change
Change-Id: I837a019708919b66b8801c37abd5aacb1d7c249f

tests/unit/java/com/android/server/ConnectivityServiceTest.java

@@ -2206,6 +2206,7 @@ public class ConnectivityServiceTest {
         deathRecipient.get().binderDied();
         // Wait for the release message to be processed.
         waitForIdle();
+        // After waitForIdle(), the message was processed and the service didn't crash.
     }
 
     @Test