Tidied up pod documentation for sub frequency.

[manwar]

Hi @rehsack

During the live session with Lance Wicks doing the weekly challenge, we found out the documentation for sub frequency was not very explicit as far as example in the doc.

https://www.youtube.com/watch?v=uM0TTQZaZ2Q

In this PR, I propose a verbose example usage of sub frequency.

Please review the PR when you get time.

I don't expect you to push a new release just for this but I would appreciate if you could please accept and merge the PR.

Many Thanks.

Best Regards,
Mohammad

[rehsack]

While I appreciate an improved example, all other given examples a way more compact.
An example with some strings and not only numbers is likely preferred, either.