vhost_task: fix vhost_task_create() documentation

Commit cb38090 ("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>

kernel/vhost_task.c

@@ -111,7 +111,7 @@ EXPORT_SYMBOL_GPL(vhost_task_stop);
  * @arg: data to be passed to fn and handled_kill
  * @name: the thread's name
  *
- * This returns a specialized task for use by the vhost layer or NULL on
+ * This returns a specialized task for use by the vhost layer or ERR_PTR() on
  * failure. The returned task is inactive, and the caller must fire it up
  * through vhost_task_start().
  */