Each Service's README should document which environment variables it uses.
each 3.0 service (including the Template Service), has a section in the README for expected environment variables.
the section should bullet point out the environment variable used
if an environment variable is only used in this Service (no other service uses it), then be sure to go into some detail about what it's used for, expected values, if it's required or any default values, etc
if an environment variable is used by the service, as well as many other services, it may make sense to link the description back to one service (e.g. Template service) that defines it in more detail.