Add javadoc for Network#netId

Bug: 151156820
Test: m doc-comment-check-docs
Change-Id: Iea34a5c58a8b77c83258a39a8a44601e21a482f5
diff --git a/core/java/android/net/Network.java b/core/java/android/net/Network.java
index 8d1ab33..c145b2b 100644
--- a/core/java/android/net/Network.java
+++ b/core/java/android/net/Network.java
@@ -61,6 +61,7 @@
 public class Network implements Parcelable {
 
     /**
+     * The unique id of the network.
      * @hide
      */
     @SystemApi