Document vec_sep2 for cli_vec(style)

[ddsjoberg]

Hello @gaborcsardi ! I love this package! 🤩

I was modifying .val function to list elements with an "or" at the end instead of "and", e.g. cli::cli_text("{.val {letters[1:3]}}"). Everything was going smoothly until I passed a vector of size two, and my "or" reverted back to "and". I was pretty confused, but after searching the internals, I found the style element vec_sep2 which quickly solved my issue.

This PR simply adds vec_sep2 to the cli_vec() help file to make it a bit easier to find for users. I think others may benefit from this as well?

(Apologies, for not re-documenting the roxygen comments....I was running into an error.)

Thank you!