Improve documentation making it easier to understand how to use interpolated variables in exec

[ndejong]

Improve documentation making it easier to understand how to use interpolated variables in exec

The documentation examples are thin-in-detail on this https://arpwitch.readthedocs.io/en/latest/docs/command-line/examples/

And there is no other reference to the variables in the documentation other than in the CLI help - thus easily missed.

See ticket #2