Without good understanding of how seed devices works it's hard to grok
some of what the code in open_seed_devices or btrfs_prepare_sprout does.

Add comments hopefully reducing some of the cognitive load.
Reviewed-by: NAnand Jain <anand.jain@oracle.com>
Signed-off-by: NNikolay Borisov <nborisov@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>