Documentation: document ublk

Add documentation for ublk subsystem. It was supposed to be documented when
merging the driver, but missing at that time.

Cc: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Richard W.M. Jones <rjones@redhat.com>
Cc: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: ZiyangZhang <ZiyangZhang@linux.alibaba.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
[axboe: correct MAINTAINERS addition]
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Documentation/block/index.rst

@@ -23,3 +23,4 @@ Block
    stat
    switching-sched
    writeback_cache_control
+   ublk

MAINTAINERS

@@ -20722,6 +20722,7 @@ UBLK USERSPACE BLOCK DRIVER
 M:	Ming Lei <ming.lei@redhat.com>
 L:	linux-block@vger.kernel.org
 S:	Maintained
+F:	Documentation/block/ublk.rst
 F:	drivers/block/ublk_drv.c
 F:	include/uapi/linux/ublk_cmd.h