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:

Task                          Description

=============================================================

Default                       Default Task Description


Argument                       Description

=============================================================

--someArgument=<int>           Argumentdescription. 

                               Can be multiline, maybe. Defaults to x.

--someOtherargument=<string>   Some other argument description

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

Start by reading the existing Argument methods and the -showdescription behavior described in the issue, then compare how task descriptions are registered and displayed. Done means arguments can carry optional help text and -showdescription presents those arguments alongside task descriptions, including multiline text and defaults.

Written by the indexing model from the issue text.