Required parameter annotation

[JosiahParry]

The plumber docs (as far as I can tell) do not show how to mark a parameter as required (or optional) without using the programmatic usage.

I would prefer to use the comment annotation as it is far easier to write and read.

Marking a parameter as optional is alluded to in the examples at https://www.rplumber.io/articles/annotations.html#equivalent-programmatic-usage-1. This is only shown for the programatic usage, however.