Add additional javadoc for VcnManager

This change adds additional detail for the VCN, describing feature set,
use cases and fail-safe modes.

Bug: 163431877
Test: Compiles, Boots (Comment-only change)
Change-Id: I1f82267c20dbbf5875ae27117debdb58f7b96e57

core/java/android/net/vcn/VcnManager.java

@@ -27,6 +27,30 @@ import android.os.RemoteException;
 /**
  * VcnManager publishes APIs for applications to configure and manage Virtual Carrier Networks.
  *
+ * <p>A VCN creates a virtualization layer to allow MVNOs to aggregate heterogeneous physical
+ * networks, unifying them as a single carrier network. This enables infrastructure flexibility on
+ * the part of MVNOs without impacting user connectivity, abstracting the physical network
+ * technologies as an implementation detail of their public network.
+ *
+ * <p>Each VCN virtualizes an Carrier's network by building tunnels to a carrier's core network over
+ * carrier-managed physical links and supports a IP mobility layer to ensure seamless transitions
+ * between the underlying networks. Each VCN is configured based on a Subscription Group (see {@link
+ * android.telephony.SubscriptionManager}) and aggregates all networks that are brought up based on
+ * a profile or suggestion in the specified Subscription Group.
+ *
+ * <p>The VCN can be configured to expose one or more {@link android.net.Network}(s), each with
+ * different capabilities, allowing for APN virtualization.
+ *
+ * <p>If a tunnel fails to connect, or otherwise encounters a fatal error, the VCN will attempt to
+ * reestablish the connection. If the tunnel still has not reconnected after a system-determined
+ * timeout, the VCN Safe Mode (see below) will be entered.
+ *
+ * <p>The VCN Safe Mode ensures that users (and carriers) have a fallback to restore system
+ * connectivity to update profiles, diagnose issues, contact support, or perform other remediation
+ * tasks. In Safe Mode, the system will allow underlying cellular networks to be used as default.
+ * Additionally, during Safe Mode, the VCN will continue to retry the connections, and will
+ * automatically exit Safe Mode if all active tunnels connect successfully.
+ *
  * @hide
  */
 @SystemService(Context.VCN_MANAGEMENT_SERVICE)