Add javadoc for Network#netId

Bug: 151156820
Test: m doc-comment-check-docs

Change-Id: I18e6022c7b0b8bf132069f33d916d1b16d53d4f0
Merged-In: Iea34a5c58a8b77c83258a39a8a44601e21a482f5
(cherry picked from commit ab23c0fef8c02a9d63b1b48628eb687f589aac72)
diff --git a/core/java/android/net/Network.java b/core/java/android/net/Network.java
index c5681cb..6f5471b 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