Clarify redis socket path format in the docs (was: Redis server with socket connection not working)

I set up redis with `{ socket = "/var/run/redis/redis.sock" }` and this is the result:
```
ssl_certificate.lua:134: get_cert_der(): auto-ssl: error fetching certificate from storage for www.example.net: failed to parse host name "/var/run/redis/redis.sock": invalid host
ssl_certificate.lua:77: issue_cert(): auto-ssl: failed to obtain lock: failed to parse host name "/var/run/redis/redis.sock"
ssl_certificate.lua:286: auto-ssl: could not get certificate for www.example.net - using fallback - failed to get or issue certificate
```

Start by locating the Redis configuration documentation referenced by the socket setting and compare its documented format with the reported error. Clarify the valid socket path syntax and ensure the documentation explains what the parser expects; done means a reader can configure a Redis socket without it being treated as a hostname.

Written by the indexing model from the issue text.