Add some help texts to recently-introduced kconfig items

Signed-off-by: Jan Engelhardt <jengelh@gmx.de>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de> (edited MACINTOSH_DRIVERS per Geert Uytterhoeven's remark)
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>