I'm starting document of the raid5-cache feature. Please note this is a
kernel doc instead of a mdadm manual, so I don't add the details about
how to use the feature in mdadm side.
Cc: NeilBrown <firstname.lastname@example.org>
Reviewed-by: Song Liu <email@example.com>
Signed-off-by: Shaohua Li <firstname.lastname@example.org>