WIP: Client-side authentication doc

- [x] Links to appropriate `/ops` docs
- [x] Explanation write-up
- [x] Ruby
- [ ] Python
- [x] Erlang
- [ ] Java
- [ ] curl
`server_cert` and `server_ca` aren't valid options. `ca_file` and `ca_path` provide the server certificate authority, which *should* work for a server cert if the server certificate is self-signed.

Broadly, TLS authentication shouldn't rely on passing certs for specific servers around as part of configuration, just certificate authority certs.