btrfs: document some invariants of seed code

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: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>