Add return type javadoc to NetworkStateSnapshot#getLegacyType

Add javadoc to document the return type is the legacy network
type in getLegacyType.

Bug: 183972826
Test: only update javadoc
Change-Id: I7757253af5955f7d489d6349c090dcba146cfd7f

core/java/android/net/NetworkStateSnapshot.java

@@ -104,7 +104,10 @@ public final class NetworkStateSnapshot implements Parcelable {
         return mSubscriberId;
     }
 
-    /** Get the legacy type of the network associated with this snapshot. */
+    /**
+     * Get the legacy type of the network associated with this snapshot.
+     * @return the legacy network type. See {@code ConnectivityManager#TYPE_*}.
+     */
     public int getLegacyType() {
         return mLegacyType;
     }