btrfs: lzo: document the compressed data format

Although it's not that complex, but such comment could still save
several minutes for newer reader/reviewer instead of inferring that from
the code.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ minor wording updates ]
Signed-off-by: David Sterba <dsterba@suse.com>