user-guide: clarify note for Runlevels/rc-update usage
user-guide: clarify note for Runlevels/rc-update usage

I found the original note a little confusing, since using rc-update will
add it to a runlevel so it *is* auto-started when the system reaches
that runlevel again, but I don't think that was the intended meaning of
'auto-start', so hopefully this makes it a little more clear.

OpenRC/openrc,OpenRC/openrc,williamh/openrc,OpenRC/openrc,williamh/openrc,williamh/openrc