docs: Add example of proc being acceptable as filter in config.include

[the-spectator]

While reading through this issue discussion, I learned that Procs can be used as filters metadata.

In this PR, I am adding an example of using a Proc with the config.include method and the :type filter accepting proc as a value.

[JonRowe]

Thanks this is documented in our feature docs but doesn't hurt to include here

[JonRowe]

Thanks again

[the-spectator]

Thank you for reviewing and merge @JonRowe 🎉