Confusion between elasticsearch and kibana SSL settings

_I wasn't sure which repo to use for this issue, but I think this is probably the best fit, even if the fix ends up elsewhere_

I've run into 2 situations recently where someone tried to us ES style`xpack.*` settings to configure SSL in Kibana.
One is [on discuss](https://discuss.elastic.co/t/unable-to-get-https-working-on-kibana/134283/2), the other was on private communications.

I'm not sure exactly where the issue lies, whether the docs for setting up TLS is Elasticsearch need to have a link to the Kibana docs, or whether the Kibana docs are hard to find, but 2 cases in as many weeks (that I've seen) implies that we should do _something_ to clarify the setup process.