Add documentation

Bug: 185121472
Test: doc-only change
Merged-In: I3c2563d4ae4e3715d0c6270344ba8f7ef067872f
Merged-In: I5fd68b209959043f6fe15ac34629c51233fea596
Change-Id: I5fd68b209959043f6fe15ac34629c51233fea596
  (cherry-picked from ag/14186082)

framework/src/android/net/NetworkProvider.java

@@ -167,7 +167,15 @@ public class NetworkProvider {
         ConnectivityManager.from(mContext).declareNetworkRequestUnfulfillable(request);
     }
 
-    /** @hide */
+    /**
+     * A callback for parties registering a NetworkOffer.
+     *
+     * This is used with {@link ConnectivityManager#offerNetwork}. When offering a network,
+     * the system will use this callback to inform the caller that a network corresponding to
+     * this offer is needed or unneeded.
+     *
+     * @hide
+     */
     @SystemApi
     public interface NetworkOfferCallback {
         /**

framework/src/android/net/NetworkScore.java

@@ -48,7 +48,14 @@ public final class NetworkScore implements Parcelable {
     })
     public @interface KeepConnectedReason { }
 
+    /**
+     * Do not keep this network connected if there is no outstanding request for it.
+     */
     public static final int KEEP_CONNECTED_NONE = 0;
+    /**
+     * Keep this network connected even if there is no outstanding request for it, because it
+     * is being considered for handover.
+     */
     public static final int KEEP_CONNECTED_FOR_HANDOVER = 1;
 
     // Agent-managed policies