btrfs: raid56: add an overview for the btrfs_raid_bio structure

btrfs: raid56: add an overview for the btrfs_raid_bio structure

The structure needs to track both the pages from higher layer bio and
internal pages, thus it can be a little complex to grasp.

Add an overview of the structure, especially how we track different
pages from higher layer bios and internal ones, to save some time for
future developers.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>