[docs]: Describe disabling vendor-data on the vendor-data page

# Documentation request
Regarding the vendor-data, the doc[1] mentions that:
[1] https://cloudinit.readthedocs.io/en/latest/explanation/vendordata.html

`1. Users have ultimate control over vendor data. They can disable its execution or disable handling of specific parts of multi-part input.`

Does it mean users could disable the call to vendor-data?
If yes, there is no detail methods provided in the doc, could you please provide the detail ways how to disable the call to vendor-data? e.g. How to config it?

# Background
This requirement comes from the below user case:

We use this datasource config on the image:
```
datasource_list: [NoCloud]

datasource:
NoCloud:
seedfrom: https://server.example.com/userdata/
```

When we do so and when cloud-init runs,
It gets the userdata metadata and then it also asks for the vendor-data..
At satellite side, we don't server anything as vendor-data, so we accepted the errors raised by that endpoint and then converted it to WARNINGS
But the end-user wants to know, if we can disable the call to the vendor-data endpoint somehow or not ..