mdadm: document some more magic environment variables.

Others are mostly for developers.

Signed-off-by: NeilBrown <neilb@suse.de>
Monitor: Stop monitoring devices that have disappeared.

If we are only monitoring a device because we found it in
/proc/mdstat, and it has been gone for 5 checks, forget
about it completely.

Signed-off-by: NeilBrown <neilb@suse.de>