F# Spec doesn't mention private representation Discriminated unions

For example, in this type:

```
type HtmlAttribute =
private | HtmlAttribute of name:string * value:string
```

What's the effect that `private` should have? I see that in C# projects, I no longer have the `NewHtmlAttribute` constructor, but I can keep doing pattern matching and create new values with `HtmlAttribute("a", "b")` in F#. Is that the intended behaviour? I was expecting to not be able to create new values using the union case constructor, and force people to use a static method that normalizes the arguments

从 F# 语言规范和 issue 中的 discriminated-union 示例开始。确定 private 对 union 表示形式的文档化影响，包括 C# 构造函数的可见性、F# 模式匹配以及 union-case 的构造；当规范明确说明预期行为时，即视为完成。

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