Add reference documentation for secret()

[Gijsreyn]

PR Summary

This pull request adds the reference documentation for the secret() function. Also, minor tweaks were made to the CLI command function to accommodate the addition of more functions without breaking the documentation.

PR Context

[Gijsreyn]

@michaeltlombardi After revisiting the examples, I have removed examples 3 and 4. Additionally, examples 1 and 2 have been rewritten, as they cannot be used in a string concatenation (still wondering why it couldn't be done even though the ARM docs also always directly use it on a property).