driver core: add more help description for "path to uevent helper"

The kernel configuration UEVENT_HELPER_PATH is a string to
the uevent helper program. Add more description about how
to disable this feature and how to enable it again during
runtime.

Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>