Unclear how to use built-in validators in System.Commandline

### Describe the issue or suggestion

https://learn.microsoft.com/en-us/dotnet/standard/commandline/how-to-customize-parsing-and-validation#validators

For the love of me I don't understand where and how to add them.

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#### Document Details

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* ID: ecee34c7-34f7-05b1-4cdd-88ce021c3fb9
* Version Independent ID: 1b860494-ce44-35a3-388f-d7f106294b8c
* Platform ID: 0348dc8c-e8eb-9438-322d-a86638a651e6
* Content: [How to customize parsing and validation in System.CommandLine - .NET](https://learn.microsoft.com/en-us/dotnet/standard/commandline/how-to-customize-parsing-and-validation#validators)
* Content Source: [docs/standard/commandline/how-to-customize-parsing-and-validation.md](https://github.com/dotnet/docs/blob/main/docs/standard/commandline/how-to-customize-parsing-and-validation.md)
* Service: **dotnet-fundamentals**
* GitHub Login: @gewarren
* Microsoft Alias: **gewarren**