Update contributing docs (#4246)

* Update ASPNetCore-Debugging.md

* Update Roslyn-Debugging.md

docs/contributing/ASPNetCore-Debugging.md

@@ -17,3 +17,4 @@ Sometimes it may be necessary to make changes in [`dotnet/aspnetcore`](https://g
 ## Notes:
 - ⚠️ Ensure you do not commit the changes to `aspnetcore-tooling/NuGet.config` & `aspnetcore-tooling/eng/Versions.props`!
 - If you're still seeing build errors after performing the above steps, you may have to temporarily modify `OldVersionUpperBound` and `NewVersion` of the first five assemblies in [AssemblyBindingRedirects.cs](https://github.com/dotnet/aspnetcore-tooling/blob/main/src/Razor/src/Microsoft.VisualStudio.RazorExtension/AssemblyBindingRedirects.cs) to match the assembly version of the aspnetcore packages above. You can find the assembly version by opening one of the packages with [ILSpy](https://github.com/icsharpcode/ILSpy/releases) or similar tool. 
+- If you find the old packages are still being used after this change, purge the nuget cache here: `~\.nuget\packages`

docs/contributing/Roslyn-Debugging.md

@@ -14,3 +14,4 @@ Sometimes it may be necessary to make changes in [`dotnet/roslyn`](https://githu
 - If you're familiar with _Visual Studio Hives_ the `dotnet/roslyn` project uses the `RoslynDev` root suffix .
 - [Building Roslyn on Windows](https://github.com/dotnet/roslyn/blob/main/docs/contributing/Building,%20Debugging,%20and%20Testing%20on%20Windows.md)
 - [Building Roslyn on Linux and Mac](https://github.com/dotnet/roslyn/blob/main/docs/infrastructure/cross-platform.md)
+- If you find the old packages are still being used after this change, purge the nuget cache here: `~\.nuget\packages`