doc: Show a command to enable IPv4 forwarding
doc: Show a command to enable IPv4 forwarding

Using QEMU host on a Zephyr application using IPv4 to access Internet
requires IPv4 forwarding, so show a command to enable IPv4 forwarding.

Signed-off-by: Stig Bjørlykke <ea07fa156e332ba20900aa10589adbee8ec38deb@nordicsemi.no>

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