TestNetworkManager: add note on kernel version expectation

Both setting the carrier up / down and creating an tun / tap interface
without carrier need TUNSETCARRIER to be present in the kernel.

Bug: 236783925
Test: documentation only change
Change-Id: Ie7f7d2c81c926adbc10b29b4b1748e47edaa4cb7
diff --git a/framework/src/android/net/TestNetworkManager.java b/framework/src/android/net/TestNetworkManager.java
index 7b18765..9cae9e6 100644
--- a/framework/src/android/net/TestNetworkManager.java
+++ b/framework/src/android/net/TestNetworkManager.java
@@ -236,6 +236,8 @@
     /**
      * Create a tap interface with or without carrier for testing purposes.
      *
+     * Note: setting carrierUp = false is not supported until kernel version 5.0.
+     *
      * @param carrierUp whether the created interface has a carrier or not.
      * @param bringUp whether to bring up the interface before returning it.
      * @hide
@@ -254,7 +256,6 @@
      * Enable / disable carrier on TestNetworkInterface
      *
      * Note: TUNSETCARRIER is not supported until kernel version 5.0.
-     * TODO: add RequiresApi annotation.
      *
      * @param iface the interface to configure.
      * @param enabled true to turn carrier on, false to turn carrier off.