[Draft] About for Possible Value

[ModProg]

This is just a draft for a possible implementation see also (#2731).

It supports:

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

and

.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")