NetworkProvider: improve documentation of unregisterNetworkOffer

Add a warning to unregisterNetworkOffer to inform users that this method
executes asynchronously.

Test: documentation only change
Change-Id: Ic4742bf570ebcd4d00b0b259c35fd46688494517
diff --git a/framework/src/android/net/NetworkProvider.java b/framework/src/android/net/NetworkProvider.java
index 4dd642e..7edcbae 100644
--- a/framework/src/android/net/NetworkProvider.java
+++ b/framework/src/android/net/NetworkProvider.java
@@ -319,6 +319,11 @@
      * if it could beat any of them, and may be advantageous to the provider's implementation that
      * can rely on no longer receiving callbacks for a network that they can't bring up anyways.
      *
+     * Warning: This method executes asynchronously. The NetworkOfferCallback object can continue
+     * receiving onNetworkNeeded and onNetworkUnneeded callbacks even after this method has
+     * returned. In this case, it is on the caller to take appropriate steps in order to prevent
+     * bringing up a network.
+     *
      * @hide
      */
     @SystemApi