qemu-iotests: 140: make description slightly more verbose

Describe in a little more detail what the test is supposed to achieve.
Signed-off-by: NSascha Silbe <silbe@linux.vnet.ibm.com>
Message-id: 1455827853-33477-3-git-send-email-silbe@linux.vnet.ibm.com
Signed-off-by: NMax Reitz <mreitz@redhat.com>

tests/qemu-iotests/140

 #!/bin/bash
 #
-# Test case for ejecting a BB with an NBD server attached to it
+# Test case for ejecting a BlockBackend with an NBD server attached to it
+#
+# Verify that the NBD server stops offering the drive when ejecting a
+# BlockDriverState tree from a BlockBackend (that is, a medium from a
+# drive) exposed via an NBD server.
 #
 # Copyright (C) 2016 Red Hat, Inc.
 #