Support about for possible_values e.g. ArgEnum

[ModProg]

Clap Version

master

Describe your use case

Sometimes the value itself my not give enough information to the user, therefore it would be helpful to be able to show and additional about in e.g. fish completions.
Fish does that for example for __fish_complete_users:

Describe the solution you'd like

Support adding abouts/documentation for the possible completion values.

For ArgEnum:

#[derive(Clone, Copy, ArgEnum)]
pub enum ShellType {
    /// the default Shell
    Bash,
    /// the friendly interactive Shell
    Fish
}

This should result in the following completions in fish for example:

Additional Context

For fish this is somewhat related to #2730

[epage]

The challenge is the derive API is built on top of the builder API, so we need to come up with a way to specify this within the builder API.

Possibly a variant of possible_values that takes a slice of tuples.

[ModProg]

Possibly a variant of possible_values that takes a slice of tuples.

Tho this is not very builder like :D but as that is probably the only information you would want to attach to a possible_value it makes sense.

I could also imagine somthing more like this (somewhat verbose if there can only be about:

.possible_value(PVal::new("bash").about("the default Shell"))
.possible_value(PVal::new("fish").about("the friendly interactive Shell"))
.possible_value(PVal::new("sh"))

Another option would be to do it via possible_value and possible_value_about:

.possible_value_about("bash","the default Shell")
.possible_value_about("fish","the friendly interactive Shell")
.possible_value("sh")

When using possible_values_about:

.possible_values_about(
    &[
        ("bash", "the default Shell"), 
        ("fish", "the friendly interactive Shell"),
        ("sh", "")
    ]
)
// or
.possible_values_about(
    &[
        ("bash", "the default Shell"), 
        ("fish", "the friendly interactive Shell")
    ]
)
.possible_value("sh")

Internally this would need to be stored in a Vec of Tuples.

[ModProg]

I drafted an Implementation in #2733 using the .possible_values_about. It works for zsh and fish, because those currently implement possible_args completion.