Adds sysfs documentation for WriteBooster entries.

Link: https://lore.kernel.org/r/1591723067-22998-1-git-send-email-asutoshd@codeaurora.orgAcked-by: Avri Altman <avri.altman@wdc.com>
Signed-off-by: Asutosh Das <asutoshd@codeaurora.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>