Add some comments to TelecomServiceImpl

Change-Id: I92d8e83a5f06a08f12e641f8cd9e910baa957caf
diff --git a/src/com/android/server/telecom/TelecomServiceImpl.java b/src/com/android/server/telecom/TelecomServiceImpl.java
index 93cfa66..f267efc 100644
--- a/src/com/android/server/telecom/TelecomServiceImpl.java
+++ b/src/com/android/server/telecom/TelecomServiceImpl.java
@@ -458,6 +458,9 @@
         }
 
         /**
+         * @return the package name of the current user-selected default dialer. If no default
+         *         has been selected, the package name of the system dialer is returned. If
+         *         neither exists, then {@code null} is returned.
          * @see android.telecom.TelecomManager#getDefaultDialerPackage
          */
         @Override