docs: document responses for insert and delete operations on permission violations

## Use-Case-One

```
mutation MyMutation {
insert_user_contacts_one(object: {contact_id: "1234567890", user_id: "2345678901"}) {
user_id
}
}
```

## Use-Case-Two

```
mutation MyMutation {
delete_user_contacts_by_pk(contact_id: "1234567890", user_id: "2345678901") {
user_id
}
}
```

## Row permissions for both use cases (insert and delete):

In _use case one_, I get a permission error back if x-hasura-user-id is not matching.

In _use case two_, I get an empty array back if x-hasura-user-id is not matching, instead of a permission error...

Hasura v1.2.0