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

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

As 67d34a6a391369269a2e5dba8a5f42cc4cd50231 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>