Show enum/known values in documentation

Since an enums values are known, these values should be displayed in the documentation when the enum is a parameter. These situations would allow this feature to work:
- The parameter is an `enum`
- The parameter has a [ValidateSet](https://docs.microsoft.com/en-us/powershell/developer/cmdlet/validateset-attribute-declaration) attribute on it
- The parameter is the 'string enum' type we use for REST APIs (getting those via reflection seen [here](https://github.com/Azure/autorest.powershell/blob/06401f99f93406125978f41d455e8aaf2d8a6c18/extensions/powershell/resources/runtime/BuildTime/Cmdlets/ExportModelSurface.cs#L91-L94))

These values would be shown in:
- Markdown documentation as `Accepted values` for the parameter descriptions (as seen for Option and Scope in [this cmdlet](https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.utility/set-alias?view=powershell-6#parameters))
- Markdown documentation in the Syntax section as `{ }` described [here](https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_command_syntax?view=powershell-6#symbols-in-syntax-diagrams)
- Note that means it no longer shows the type of the parameter in this syntax. Meaning `[-Option ]` becomes `[-Option {First | Second | Third}]`.
- Complex type documentation in the exports and markdown