docs/resource/aws_transfer_server: Add VPC for endpoint_type to tra…

…nsfer_server (#15373)

Document `VPC` value for endpoint_type attribute in transfer_server resource.

I put `VPC_ENDPOINT` in parentheses because it is no longer possible to create this type of Transfer servers in AWS dashboard. It is only accessible via the API, so I believe it will be eventually deprecated, but I didn't find any information to support that.

website/docs/r/transfer_server.html.markdown

@@ -68,7 +68,7 @@ POLICY
 The following arguments are supported:
 
 * `endpoint_details` - (Optional) The virtual private cloud (VPC) endpoint settings that you want to configure for your SFTP server. Fields documented below.
-* `endpoint_type` - (Optional) The type of endpoint that you want your SFTP server connect to. If you connect to a `VPC_ENDPOINT`, your SFTP server isn't accessible over the public internet. If you want to connect your SFTP server via public internet, set `PUBLIC`.  Defaults to `PUBLIC`.
+* `endpoint_type` - (Optional) The type of endpoint that you want your SFTP server connect to. If you connect to a `VPC` (or `VPC_ENDPOINT`), your SFTP server isn't accessible over the public internet. If you want to connect your SFTP server via public internet, set `PUBLIC`.  Defaults to `PUBLIC`.
 * `invocation_role` - (Optional) Amazon Resource Name (ARN) of the IAM role used to authenticate the user account with an `identity_provider_type` of `API_GATEWAY`.
 * `host_key` - (Optional) RSA private key (e.g. as generated by the `ssh-keygen -N "" -m PEM -f my-new-server-key` command).
 * `url` - (Optional) - URL of the service endpoint used to authenticate users with an `identity_provider_type` of `API_GATEWAY`.