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>