Documentation incorrectly states that the room list always returns an empty list if disabled, while federation actually returns a 403 error

This issue has been migrated from [#13102](https://github.com/matrix-org/synapse/issues/13102).

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### Description

https://github.com/matrix-org/synapse/blob/develop/docs/usage/configuration/config_documentation.md?plain=1#L3337-L3339 states that both internally and externally synapse will return an empty list on _ALL_ queries.

However https://github.com/matrix-org/synapse/blob/c52abc1cfdd9e5480cdb4a03d626fe61cacc6573/synapse/federation/transport/client.py#L491 (and testing it via fed) states that actually M_FORBIDDEN and a 403 error is returned.

### Steps to reproduce

- Access a remote room list via fed where the remote server runs default config.

### Homeserver

Not relevant. This is a docs issue.

### Synapse Version

1.61.0

### Installation Method

Docker (matrixdotorg/synapse)

### Platform

Not relevant. This is a docs issue.

### Relevant log output

```shell
Not relevant. This is a docs issue.
```

### Anything else that would be useful to know?

_No response_