Since `ExpressibleByArgument` already maintains a list of
enumerable values for an argument, we can extend this to serve
as an ordered list for a new dictionary property that maps the
value name to its description, if applicable. The new property
is a static variable on `ExpressibleByArgument` labelled
`allValueDescriptions`.
If the description string for a value is the same as the value
string, it's assumed that the description is not implemented.
The new value strings are used in the help screen, in the
dump-help JSON output, and in the generated manual.
Nate Cook
Rauhul Varma
Bri Peticca