Add an example on how to provide additional help for each item in an enum

[bkhouri]

As I was reading the Documentation, I was unclear to me how we an additional help message for each item in an enum. It would be great if the documentation provided an example of such. Maybe in the `Customizing Help for Commands* or a new document all together.

[bripeticca]

I've done some work to support this here: https://github.com/apple/swift-argument-parser/pull/647/files
But this does indeed need documentation! :)

[louisunlimited]

Hi @bkhouri @bripeticca I took a stab at this, lmk if it looks good or needs fixing :))