[docs] No mention how to skip enum variants on the ValueEnum derive macro

[dnaka91]

Please complete the following tasks

• I have searched the discussions
• I have searched the open and rejected issues

Clap Version

4.0.7

Describe your use case

I was looking for a way to skip a variant from an enum, when using the ValueEnum derive macro. Turns out there actually IS a way, but it's not mentioned anywhere in the reference in a direct way.

Simple example:

#[derive(Debug, ValueEnum)]
struct Format {
    Text,
    Json,
    #[value(skip)]
    None, // handled by another flag manually
}

The hide attribute is mentioned (somewhat), as one can check the available functions on the PossibleValue struct, but it's not the same as it only hides it in the docs. Only with #[value(skip)] it's fully omitted and becomes an impossible value to create through the CLI.

The skip attribute is only mentioned in a few places in the derive reference:

• In the command attributes, as part of the Subcommand specific attributes: https://docs.rs/clap/4.0.7/clap/_derive/index.html#command-attributes
• In the arg group attributes: https://docs.rs/clap/4.0.7/clap/_derive/index.html#arggroup-attributes
• In the arg attributes, but only for typical fields and saying that it uses Default::default() or the given expression, so it doesn't look applicable for enums: https://docs.rs/clap/4.0.7/clap/_derive/index.html#arggroup-attributes
• The overview talks about skipping of non-unit enum variants, but doesn't tell how: https://docs.rs/clap/4.0.7/clap/_derive/index.html#overview

Describe the solution you'd like

There is already a ValueEnum section for attributes in the docs: https://docs.rs/clap/4.0.7/clap/_derive/index.html#valueenum-attributes

This one only needs an additional entry for the skip attribute, describing that it can be used to omit variants from the generated implementation.

Alternatives, if applicable

No response

Additional Context

No response