docs: sysfs-block: document virt_boundary_mask

/sys/block/<disk>/queue/virt_boundary_mask is completely undocumented.
Document it.
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20211209003833.6396-7-ebiggers@kernel.orgSigned-off-by: Jens Axboe <axboe@kernel.dk>

Documentation/ABI/stable/sysfs-block

@@ -544,6 +544,18 @@ Description:
 		CONFIG_BLK_DEV_THROTTLING_LOW is enabled.
 
 
+What:		/sys/block/<disk>/queue/virt_boundary_mask
+Date:		April 2021
+Contact:	linux-block@vger.kernel.org
+Description:
+		[RO] This file shows the I/O segment memory alignment mask for
+		the block device.  I/O requests to this device will be split
+		between segments wherever either the memory address of the end
+		of the previous segment or the memory address of the beginning
+		of the current segment is not aligned to virt_boundary_mask + 1
+		bytes.
+
+
 What:		/sys/block/<disk>/queue/wbt_lat_usec
 Date:		November 2016
 Contact:	linux-block@vger.kernel.org