docs: note about select in kconfig-language.txt

A warning note from Sam Ravnborg about kconfig's select evilness,
dependencies and the future (slightly corrected).

Signed-off-by: Jarek Poplawski <jarkao2@o2.pl>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Roman Zippel <zippel@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>