Document loss of performance in RAID-0 when a disk is larger than others

See https://raid.wiki.kernel.org/index.php/What_is_RAID_and_why_should_you_want_it%3F#RAID0.2FStripe_Mode for the short explanation that we should integrate in Chapter 12 at the appropriate place.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information