AppContext no longer correctly uses registry

In the description for setting a switch it refers to using the registry. However this functionality was [broken](https://support.microsoft.com/en-us/help/4036977/fix-appcontext-switch-overrides-are-not-applied-to-applications-that-r) in 4.7 and hasn't been fixed as of 4.7.2. So the registry cannot be used to change a switch, only the configuration file will work.

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#### Document Details

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* ID: 06ff8e85-e420-bd37-7583-3d7ff3605156
* Version Independent ID: 4d662dc9-4702-ca4d-8a4e-d422cdc057a4
* Content: [AppContext Class (System)](https://docs.microsoft.com/en-us/dotnet/api/system.appcontext?view=netframework-4.7.2)
* Content Source: [xml/System/AppContext.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System/AppContext.xml)
* Product: **dotnet-api**
* GitHub Login: @rpetrusha
* Microsoft Alias: **ronpet**