Clarify docs relating to custom ports

[mheadd]

There is a new feature of Cloud Foundry that allows users to set a custom port(s) for their app to listen on, rather than the default port used when apps are deployed.

Our documentation also currently has details about using custom ports with Docker containers on internal routes.

We should clarify and organize this information in our docs so that customers have a complete picture of how ports are assigned during a standard deploy, and options available to set. custom port if needed.