The documentation doesn't mention that `matrix_rtc` only works when msc4143_enabled is enabled.

### Description

https://element-hq.github.io/synapse/latest/usage/configuration/config_documentation.html?highlight=rtc#matrix_rtc should call this out.

### Steps to reproduce

N/A docs

### Homeserver

N/A docs

### Synapse Version

1.144.0

### Installation Method

Other (please mention below)

### Database

N/A

### Workers

Single process

### Platform

N/A

### Configuration

N/A

### Relevant log output

```shell
N/A
```

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

N/A