kbuild: add distclean info to 'make help' and more details for 'clean'

Add distclean info, that was previously missing, to 'make help'.
Also add a few more details to the 'make clean' help text.

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>