btrfs: add a comment explaining the data flush steps

The data flushing steps are not obvious to people other than myself and
Chris.  Write a giant comment explaining the reasoning behind each flush
step for data as well as why it is in that particular order.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>