Provide ability to create documentation for Arguments similar to Tasks

Hi,

as described in this SO question: https://stackoverflow.com/questions/45413524/how-can-cake-build-arguments-be-documented it would be great to be able to add documentation for Arguments as it is currently possible for Tasks by using the Description("...") method.

So e.g. executing the build script with the -showdescription parameter could show the described targets as well as the available arguments with an optional help text:

For registering a help text with an argument an additional parameter could be introduced to the already existing Argument methods. E.g.

Argument("someArgument", "x", "Argument description. Can be multiline, maybe.")

Or some other way:

var argValue = ArgumentBuilder("someArgument", "x")
                              .WithHelpText("Argument description. Can be multiline, maybe.")
                              .GetValue()

What do you think?

Best regards, Philipp