west: runners: nrf: Document why the network core is recovered first

The network core needs to be recovered first due to the fact that
recovering it erases both the network and application cores' flash
memory. This means that the little flash image that is programmed during
the recover operation would be lost for the app core if we inverted the
order.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>