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.

首先定位 DefaultSentenceBuilder 和 SentenceBuilder 的声明，并检查 SentenceBuilder.Factory 的使用方式。将提议的继承方案与把默认实现移入 SentenceBuilder 的方案进行比较，然后确定应支持哪些自定义行为以及应如何对其进行验证。

由索引模型根据 Issue 内容生成。