native doc: minor improvement in ethernet driver
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 <52be3174920ed06cbdfbcf9622a1a6c68e942617@oticon.com>

ldts/zephyr,Vudentz/zephyr,punitvara/zephyr,mbolivar/zephyr,Vudentz/zephyr,zephyriot/zephyr,explora26/zephyr,kraj/zephyr,galak/zephyr,ldts/zephyr,ldts/zephyr,punitvara/zephyr,kraj/zephyr,explora26/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,punitvara/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,kraj/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,mbolivar/zephyr,galak/zephyr,explora26/zephyr,ldts/zephyr,mbolivar/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,explora26/zephyr,explora26/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,Vudentz/zephyr,zephyriot/zephyr,finikorg/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,zephyriot/zephyr,punitvara/zephyr