silence xmldoc warnings for non-nuget projects

src/csharp/Grpc.Auth/Grpc.Auth.csproj

src/csharp/Grpc.Core.Testing/Grpc.Core.Testing.csproj

src/csharp/Grpc.Core/Common.csproj.include

src/csharp/Grpc.Core/Grpc.Core.csproj

src/csharp/Grpc.HealthCheck/Grpc.HealthCheck.csproj

src/csharp/Grpc.Reflection/Grpc.Reflection.csproj