List sample path for each Environment.SpecialFolder field and update the description of the fields for targeting to multiple platforms.

[Current document for `Environment.SpecialFolder`](
https://docs.microsoft.com/zh-cn/dotnet/api/system.environment.specialfolder?view=netcore-2.0) only lists the meaning of each field on Windows. There're several problems since .NET Core targets multiple platform:

1. The field description is not accuracy. For instance, `ProgramFilesX86` indicates nothing for Linux and Mac. We need to let user know what platforms are supported for each field.
2. When it comes only to Windows, we probably are able to guess what these fields stand for. However, .NET core is now targeting to multiple platforms. Without explicit examples, people are confusing about the value of the fields. It would be great if we list sample path for each `Environment.SpecialFolder` field to make it more intuitive. It'll save a lot of time for developers.