scsi: sd: Documentation: add sd-parameters.txt

Add a file for documenting SCSI sd module parameters and describe the
cache_type setting.

[mkp: tweaked text a bit]
Signed-off-by: Weiping Zhang <zhangweiping@didichuxing.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Documentation/scsi/sd-parameters.txt

+Linux SCSI Disk Driver (sd) Parameters
+======================================
+
+cache_type (RW)
+---------------
+Enable/disable drive write & read cache.
+
+ cache_type string          | WCE RCD | Write cache | Read cache
+----------------------------+---------+-------------+------------
+ write through              | 0   0   | off         | on
+ none                       | 0   1   | off         | off
+ write back                 | 1   0   | on          | on
+ write back, no read (daft) | 1   1   | on          | off
+
+To set cache type to "write back" and save this setting to the drive:
+
+  # echo "write back" > cache_type
+
+To modify the caching mode without making the change persistent, prepend
+"temporary " to the cache type string. E.g.:
+
+  # echo "temporary write back" > cache_type