Correct description of PathTooLongException

# Update description of PathTooLongException

## General

In member documentation, almost all descriptions of the condition under which PathTooLongException is thrown mention the 260 character limit in the .NET Framework 4.6.1 and earlier versions. This needs to be changed, since the .NET Framework supports long paths. For [Directory.GetParent](https://review.docs.microsoft.com/dotnet/api/system.io.directory.getparent), it was changed to "The specified path, file name, or both exceed the system-defined maximum length. For more information, see the topic." in PR dotnet/docs#4283.

In addition, the PathTooLongException topic itself is too .NET Framework-centric. It fails to mention the size of supported paths on Linux and MacOS.

//cc @mairaw @JeremyKuhne @pjanotti