Improve documentation of addEthernetStateListener

The existing documentation was not clear at all about how this API
behaves.

Test: TH
Change-Id: I408f32c1e7b6c910aaaa197b6774865e79a03579
diff --git a/framework-t/src/android/net/EthernetManager.java b/framework-t/src/android/net/EthernetManager.java
index b8070f0..719f60d 100644
--- a/framework-t/src/android/net/EthernetManager.java
+++ b/framework-t/src/android/net/EthernetManager.java
@@ -642,7 +642,14 @@
     }
 
     /**
-     * Listen to changes in the state of ethernet.
+     * Register a IntConsumer to be called back on ethernet state changes.
+     *
+     * <p>{@link IntConsumer#accept} with the current ethernet state will be triggered immediately
+     * upon adding a listener. The same callback is invoked on Ethernet state change, i.e. when
+     * calling {@link #setEthernetEnabled}.
+     * <p>The reported state is represented by:
+     * {@link #ETHERNET_STATE_DISABLED}: ethernet is now disabled.
+     * {@link #ETHERNET_STATE_ENABLED}: ethernet is now enabled.
      *
      * @param executor to run callbacks on.
      * @param listener to listen ethernet state changed.