kconfig: document Kconfig source file comments

I saw this type of Kconfig construct on LKML:

config SYMBOOL
	#bool "prompt string"
	default y

and wondered what it does.  Then I wondered if '#' comments are
even documented.  They aren't, so add a little doc for that.

Ah, good.  kconfig says:
arch/x86/Kconfig:2942:warning: config symbol defined without type

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>