ubifs: Document sysfs nodes

Add documentation for the new sysfs nodes

 /sys/fs/ubifs/ubiX_Y/errors_magic
 /sys/fs/ubifs/ubiX_Y/errors_node
 /sys/fs/ubifs/ubiX_Y/errors_crc

Signed-off-by: Stefan Schaeckeler <sschaeck@cisco.com>
Signed-off-by: Richard Weinberger <richard@nod.at>