Note about box creation

The `vagrant-cloud` post-processor does not create the desired Vagrant
Cloud box for you. Packer assumes the Vagrant Cloud box already exists
and will only push new box versions for you. It is up to the user to
ensure the Vagrant Cloud box exists beforehand. This note should make it
clear to users that Packer does not create the box.

website/pages/docs/post-processors/vagrant-cloud.mdx

@@ -47,6 +47,12 @@ Here is an example workflow:
 7.  The upload is verified
 8.  The version is released and available to users of the box
 
+~> The Vagrant Cloud box (`hashicorp/foobar` in this example) must already
+exist. Packer will not create the box automatically. If running Packer in
+automation, consider using the
+[Vagrant Cloud API](https://www.vagrantup.com/docs/vagrant-cloud/api.html)
+to create the Vagrant Cloud box if it doesn't already exist.
+
 ## Configuration
 
 The configuration allows you to specify the target box that you have access to
@@ -56,7 +62,8 @@ on Vagrant Cloud, as well as authentication and version information.
 
 - `box_tag` (string) - The shorthand tag for your box that maps to Vagrant
   Cloud, for example `hashicorp/precise64`, which is short for
-  `vagrantcloud.com/hashicorp/precise64`.
+  `vagrantcloud.com/hashicorp/precise64`. This box must already exist in
+  Vagrant Cloud. Packer will not create the box automatically.
 
 - `version` (string) - The version number, typically incrementing a previous
   version. The version string is validated based on [Semantic