Add debug logs to figure out why an event was filtered (#14095)

Spawned while investigating https://github.com/matrix-org/synapse/issues/13944

This way we might get some more context whenever an `403 Forbidden - body: {"errcode":"M_FORBIDDEN","error":"You don't have permission to access that event."}` error is produced.

`log_config.yaml`
```yaml
loggers:
    synapse:
        level: INFO

    synapse.visibility:
        level: DEBUG
```