Example for dynamic ValidateSet values

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Issue Details

Version(s) of document impacted

• Impacts 6.1 document
• Impacts 6.0 document
• Impacts 5.1 document
• Impacts 5.0 document
• Impacts 4.0 document
• Impacts 3.0 document

Reason(s) for not selecting all version of documents

• The documented feature was introduced in selected version of PowerShell
• This issue only shows up in selected version of the document

In reference/6/Microsoft.PowerShell.Core/About/about_Functions_Advanced_Parameters.md there is no example of ValidateSet attribute with dynamic values.
This was introduced in PowerShell 6.0 as per PowerShell/PowerShell#3784 (discussed in PowerShell/PowerShell#3744).