Please complete the following tasks

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

Clap Version

master

Describe your use case

Command line options with a default_missing_value can be used as flags and then take on a specified default value. This behavior is quite useful. However, the user discoverability of it seems to be non-existent? As in, based on the help output a user couldn't know whether an option has a default_missing_value. E.g.:

Usage: clap-6109 [OPTIONS]

Options:
      --color [<COLOR>]  [default: auto] [possible values: auto, always, never]
  -h, --help             Print help
  -V, --version          Print version

#!/usr/bin/env nargo
---
[dependencies]
clap = { version = "4", features = ["derive"] }
---

use clap::Parser;

#[derive(Parser, Debug)]
#[command(version, about, long_about = None)]
struct Cli {
    #[arg(
        long,
        value_parser = ["auto", "always", "never"],
        default_value = "auto",
        default_missing_value = "always",
        num_args = 0..=1,
    )]
    color: String,
}

fn main() {
    let cli = Cli::parse();
    println!("{cli:#?}");
}

Describe the solution you'd like

Add default missing values to the generated help message. If included/displayed the same way as default values, it would look like:

Usage: clap-6109 [OPTIONS]

Options:
      --color [<COLOR>]  [default: auto] [default missing value: always] [possible values: auto, always, never]
  -h, --help             Print help
  -V, --version          Print version

Potentially, this should be behind a flag and not the default behavior to include these. Moreover, I have no preference on the format and welcome discussion about it. It seems sensible enough to adopt the already used format for this, hence the proposal.

Alternatives, if applicable

The only way I see a user discovering this behavior is through the help message

Additional Context

This is my first feature request, please point out any mistakes I made 🥺 .