Why is DefaultSentenceBuilder private?

Is there a reason that `DefaultSentenceBuilder` is marked `private`? It would be much easier to customize `SentenceBuilder` if the class were public and could be inherited.
```csharp
public class MySentenceBuilder: DefaultSentenceBuilder
{
public override Func UsageHeadingText => () => "Usage:";
}

SentenceBuilder.Factory = () => new MySentenceBuilder();
```

Another possibility would be to make `SentenceBuilder` non-abstract, move the default implementations there from `DefaultSentenceBuilder`, and then get rid of `DefaultSentenceBuilder` altogether.

Start by locating the declarations of DefaultSentenceBuilder and SentenceBuilder and reviewing how SentenceBuilder.Factory is used. Compare the proposed inheritance approach with moving default implementations into SentenceBuilder, then establish which customization behavior should be supported and how it would be validated.

Written by the indexing model from the issue text.