Document better how this method works on different platforms

@andre-ss6 commented on [Wed Jun 19 2019](https://github.com/dotnet/dotnet-api-docs.pt-br/issues/14)

Currently this method documentation for .Net Core is the same as the .Net Framework 4.8, which [obviously] seem to have been written with a Windows-only usage in mind.

It's not clear what happens on Unix when the three mentioned checks fail and then it's even less clear what happens when it falls under the fourth point. I guess the Windows directory isn't returned under Unix?

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* ID: a81b4c67-7b0d-56c2-eccc-ef34b4ed2dca
* Version Independent ID: c05a5cc0-6c59-7954-eb46-e162419a5126
* Content: [Path.GetTempPath Method (System.IO)](https://docs.microsoft.com/en-us/dotnet/api/system.io.path.gettemppath?view=netcore-2.2)
* Content Source: [xml/System.IO/Path.xml](https://github.com/dotnet/dotnet-api-docs.pt-br/blob/live/xml/System.IO/Path.xml)
* Product: **dotnet-api**
* Technology: **system.io**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**