doc: kconfig: Explain the '<type> <prompt>' shorthand
doc: kconfig: Explain the '<type> <prompt>' shorthand

The board porting guide assumed that people were already familiar with
the Kconfig 'bool "foo"' shorthand for giving a symbol's type and prompt
at the same time. I got an email pointing out that it isn't obvious.

Explain the shorthand, and also mention that it's the preferred style in
Zephyr.

Signed-off-by: Ulf Magnusson <01743f1c1deb1fe75271d62edb65757d98ac0bcb@nordicsemi.no>

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