CORS explicit policy configuration

[pranavkm]

Need to add a snippet that indicates how the application needs to be configured for the breaking change made as part of dotnet/aspnetcore#3376 . Relevant doc: https://docs.microsoft.com/en-us/aspnet/core/security/cors?view=aspnetcore-2.1#set-the-allowed-request-headers

[Tornhoof]

If you update these docs, please use e.g. HeaderNames.CacheControl instead of Cache-Control directly. This should reduce copy/paste errors.