Documented return value of getRestrictBackgroundStatus().

Change-Id: I2c15a6d5e23cbc3c9b59c975196c1c04c1294471
Fixes: 28775490
diff --git a/core/java/android/net/ConnectivityManager.java b/core/java/android/net/ConnectivityManager.java
index 43d9bf3..a45e6f5 100644
--- a/core/java/android/net/ConnectivityManager.java
+++ b/core/java/android/net/ConnectivityManager.java
@@ -3362,6 +3362,10 @@
     /**
      * Determines if the calling application is subject to metered network restrictions while
      * running on background.
+     *
+     * @return {@link #RESTRICT_BACKGROUND_STATUS_DISABLED},
+     * {@link #RESTRICT_BACKGROUND_STATUS_ENABLED},
+     * or {@link #RESTRICT_BACKGROUND_STATUS_WHITELISTED}
      */
     public @RestrictBackgroundStatus int getRestrictBackgroundStatus() {
         try {