kconfig: document oldnoconfig to what it really does in conf.c

As 67d34a6a said, the make target
'oldnoconfig' is a misnomer. It doesn't set new symbols to 'n', but
instead sets it to their default values.

This patch fixes the document in conf.c, and will submit another patch
to replace 'oldnoconfig' to 'olddefconfig'

Signed-off-by: Adam Lee <adam8157@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>