block: null: Add documentation for "zone_nr_conv" param

zone_nr_conv module parameter was introduced by a commit
ea2c18e1("null_blk: Add conventional zone configuration for zoned
support").

This patch describes "zone_nr_conv" module parameter to the document.
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Documentation/block/null_blk.txt

@@ -93,3 +93,7 @@ zoned=[0/1]: Default: 0
 
 zone_size=[MB]: Default: 256
   Per zone size when exposed as a zoned block device. Must be a power of two.
+
+zone_nr_conv=[nr_conv]: Default: 0
+  The number of conventional zones to create when block device is zoned.  If
+  zone_nr_conv >= nr_zones, it will be reduced to nr_zones - 1.