Add doc comments to variants of the arg enum

[sorairolake]

This will display the value description in some shells (fish/zsh) completions (see PossibleValue::help()).
Also, this will show in long help from clap v4 (see clap-rs/clap#3312).

[ducaale]

I can't think of an arg enum that would benefit from documenting its variants besides --pretty. In #250, I've added the following long-help to the --pretty flag:

--pretty <STYLE>
    Controls output processing. Possible values are:

        all      (default) Enable both coloring and formatting
        colors   Apply syntax highlighting to output
        format   Pretty-print json and sort headers
        none     Disable both coloring and formatting

    Defaults to "format" if the NO_COLOR env is set and to "none" if stdout is not tty.

I can update #250, so possible args for --pretty are documented in shell completions. Do you have any other flags in mind?

Edit: Done in 5b8d663

[sorairolake]

Other than this, I think either is fine. Because I think the meaning of the value can be understood from the flag name.