net: netconsole: Add description for 'netconsole_target's extended attribute

Fixes the following W=1 kernel build warning(s):

 drivers/net/netconsole.c:104: warning: Function parameter or member 'extended' not described in 'netconsole_target'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20201102114512.1062724-29-lee.jones@linaro.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>