[enhancement]: x-tensible user-data

# Enhancement

Document how custom / experimental / non-standard items can be added to user data without triggering schema errors, e.g. using an `x-` prefix:

```
x-mystuff:
data: whatever
```

Start by locating the user-data schema documentation and any existing guidance for custom or experimental keys. Verify the x- prefixed example against the documented validation behavior; done means users can follow the example without schema errors.

Written by the indexing model from the issue text.