[DOCS]: Add security and comms info to Ingest Manager doc

[jmikell821]

Per request from Elastic Security product management, we need to update the Config Settings topic to include some additional security and comms information, as specified in this Google doc.

Copied and pasted as follows:

How are communications secured between endpoint and agent?
Endpoint connects to the agent over loopback TLS on port 6788.  Endpoint validates that the agent has root (Linux and macOS) or SYSTEM (Windows) permissions.  In future versions, the endpoint will validate the digital signature of the agent, and vice versa.

How are communications secured between agent and kibana?
Agent connects to Kibana over TLS and authenticates the certificate presented by Kibana.  Agent then provides an ApiKey as an authentication token, which Kibana validates.

What ES and Kibana ports need to be accessible?
The configuration file generated by the Ingest Manager app already contains the correct Elasticsearch address and port for your setup. If you run everything locally, the address is 127.0.0.1:9200. If you use our hosted Elasticsearch Service on Elastic Cloud, the address corresponds to the Elasticsearch endpoint URL that is listed under Endpoints as described in Work with Elasticsearch.  If not running in the cloud, both the Kibana and the Elasticsearch HTTPS ports must be accessible; by default these are 5601 and 9200 respectively.

We can discuss further during our sync on Thursday.

[dedemorton]

Closed by #238