Putting a backtick into the variable `matrix_synapse_macaroon_secret_key`
leads to an error when starting `matrix-synapse.service`, so the service
can't start. Error Message:

```
[...]
[...] docker[19227]: yaml.scanner.ScannerError: while scanning a double-quoted scalar
[...] docker[19227]:   in /data/homeserver.yaml, line 451, column 29
[...] docker[19227]: found unknown escape character '`'
[...] docker[19227]:   in /data/homeserver.yaml, line 451, column 37
[...] docker[19227]: Traceback (most recent call last):
[...] docker[19227]:   File /start.py, line 66, in <module>
[...]
```

Therefore, added comments that the secrets mustn't contain quotes or
backticks.