Update dotnet test integration doc to point out to use of Directory.Build.props (#44412)

* Update dotnet test integration doc to point out to use of Directory.Build.props

* Update docs/core/testing/unit-testing-platform-integration-dotnet-test.md

---------

Co-authored-by: Bill Wagner <[email protected]>

Author: Youssef1313

docs/core/testing/unit-testing-platform-integration-dotnet-test.md

@@ -169,3 +169,6 @@ Or in project file:
 
 </Project>
 ```
+
+> [!IMPORTANT]
+> All examples above add properties like `EnableMSTestRunner`, `TestingPlatformDotnetTestSupport`, and `TestingPlatformCaptureOutput` in the csproj file. However, it's highly recommended that you set these properties in `Directory.Build.props`. That way, you don't have to add it to every test project file, and you don't risk introducing a new project that doesn't set these properties and end up with a solution where some projects are VSTest while others are Microsoft.Testing.Platform, which may not work correctly and is unsupported scenario.