native doc: minor improvement in ethernet driver

Refer to the samples/net/eth_native_posix sample app
and mention that this driver only works on Linux
with admin rights

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>

boards/posix/native_posix/doc/board.rst

@@ -396,6 +396,11 @@ The following peripherals are currently provided with this board:
   be created if needed. The IP address configuration can be specified for each
   network interface instance.
   See :option:`CONFIG_ETH_NATIVE_POSIX_SETUP_SCRIPT` option for more details.
+  The :ref:`eth-native-posix-sample` sample app provides
+  some use examples and more information about this driver configuration.
+
+  Note that this device can only be used with Linux hosts, and that the user
+  needs elevated permissions.
 
 Shell support
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