Clarify Form documentation to explain form co-owners can't delete responses

### Description

Contacts added as co-owners of a form are not able to delete form responses.

### Steps to reproduce

1. Create owned form
2. Add contact as co-owner
3. Form responses are submitted
4. Only the original owner can delete form responses

### Relevant logs

```shell

```

### Expected behavior

The fact that new owners cannot delete responses should be clarified in [the user guide](https://docs.cryptpad.org/en/user_guide/apps/form.html#roles).

### Which operating system are you using?

_No response_

### What browsers are you seeing the problem on?

_No response_

### Extensions

n/a

### Version

2026.2.0