vhost_task: fix vhost_task_create() documentation

Commit cb380909 ("vhost: return task creation error instead of NULL")
changed the return value of vhost_task_create(), but did not update the
documentation.

Reflect the change in the documentation: on an error, vhost_task_create()
returns an ERR_PTR() and no longer NULL.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20250327124435.142831-1-sgarzare@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>