Testing old solutions using VSCode reveals mismatch in System.IO.Path.GetExtension

I have been trying to find an issue in some legacy code written by a dev at my company in 2012. I prefer to use VSCode and assumed that, as the doc suggests, the System.IO.Path.GetExtension method works the same in the versions listed in the doc.

But, there is a big difference:
.NET Core v2.1 (and possibly others) does not lineup with the functionality of checking for invalid characters in the filename. .NET Core will accept "<1235.pdf>.pdf" as a filename and extract ".pdf" from it.
.NET Framework, however, throws an ArgumentException because the filename contains invalid characters "<" and ">". Both versions are fine, but it did make finding the issue in this legacy code a hassle.

Please update the doc or fix the discrepancy so that devs in the future won't be so confused.
Note: Great work on the .NET Core and Framework!! I'm a C# dev and it is my favorite desktop application development language as of right now, especially when paired with Visual Studio.

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* ID: 00371794-2f55-5e6d-5646-e44474ce93f4
* Version Independent ID: f887b103-6668-321b-516a-50daf422a533
* Content: [Path.GetExtension Method (System.IO)](https://docs.microsoft.com/en-us/dotnet/api/system.io.path.getextension?view=netframework-3.5)
* Content Source: [xml/System.IO/Path.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.IO/Path.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**