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 <Ulf.Magnusson@nordicsemi.no>