Incomplete documentation on optional parameters with deep linking and path

Current Behavior

According to tutorials and documentation I managed to gather online, this piece of code should allow for an optional parameter param2, but it seems that it doesn't work and the path is never matched.

 ex: {
        screen: Screen,
      },
    path: "ex/:param1(/:param2)",
  },```

### Expected Behavior

I hoped here that different paths would be matched like:
- app://ex/p1
- app://ex/p1/p2

Both paths should have been matched according to my understanding of the `path` keyword (almost absent from the documentation, hence the issue).

### Your Environment

| software         | version
| ---------------- | -------
| react-navigation | 4.0.10
| react-native     | 0.61.3
| node             | n/a
| npm or yarn      | n/a

從 issue 中描述的 deep-linking 文件和 path 設定範例開始。檢查 react-navigation 4.0.10 如何記錄選用參數，並驗證 ex/p1 和 ex/p1/p2 路徑。完成標準是文件清楚地說明支援的語法和預期的比對行為。