Adds UFS sysfs documentation for new entry wb_on.

[mkp: fix doc formatting]

Link: https://lore.kernel.org/r/20210119163847.20165-3-huobean@gmail.comSigned-off-by: Bean Huo <beanhuo@micron.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

fix format