Add explainer on transport trust config to docs

[tvernum]

We don't have a clear explainer about why we mandate transport TLS (xpack.security.transport.ssl.enabled), why it's important, and how you should think about it.

We have guides on how to set it up, and if people follow those guides then they will have an appropriate configuration, but we don't explain why that's the correct setup and why other configurations might not be.

I wrote up a first pass on the forums: https://discuss.elastic.co/t/need-help-in-configuring-tls-ssl-for-elk-with-3rd-party-certificates-comodo/259745/2

I think we need to come up with a more polished version of that explanation and slot it into the security setup documentation (probably as a separate page for "advanced users" or something like that - I don't think we want to push that level of detail onto everyone that tries to configure ES for the first time).

[elasticmachine]

Pinging @elastic/es-docs (Team:Docs)

[elasticmachine]

Pinging @elastic/es-security (Team:Security)