init.h: add some more documentation to __ref* tags

The __ref* tags may have been confusing for new kernel
developers (I was confused by them for sure) so adding a few
more sentences to comment to clear things up for people who
see those for the first time.

Signed-off-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>