Make the functionality of the applications/environment configuration clearer

[javorszky]

We've run into issues where folks aren't sure how the envionment variables are being processed in Unit. The part of the documentation is this: https://unit.nginx.org/configuration/#applications

What's there:

environment -- String-valued object; environment variables to be passed to the app.

What's missing:

• does not spell out that Unit passes the available and existing environment variables to the appliaction as-is without modification even if no environment config object is specified
• anything specified in the environment object gets merged to / added to the existing environment variables
• if an environment variable already exists on the system, and gets declared in the object, the object's value gets passed to the application