Document how to provide memberwise initializers for Parsable* types

As pointed out in #120, there isn't any documentation describing how to manually create instances of `ParsableArguments` or `ParsableCommand` types, and the error message when using one created from calling the default init (e.g. `let command = MyCommand()`) indicates that the only way to create one is through parsing.

Instead, the [Manual Parsing and Testing](https://github.com/apple/swift-argument-parser/blob/master/Documentation/06%20Manual%20Parsing%20and%20Testing.md) guide should cover adding a memberwise init as a way to create testable instances of the type, and the error should be updated to indicate that possibility.

Start with Documentation/06 Manual Parsing and Testing.md and review the existing guidance for manually creating ParsableArguments or ParsableCommand instances. Then locate the error produced after calling a type's default init, update the documentation and wording to cover memberwise initializers, and verify the examples and error behavior.

Written by the indexing model from the issue text.