Document thread-safety requirements on loggers

As far as I can tell, MSBuild will always call logger methods from a single thread (either the dedicated logging thread when `LoggerMode.Asynchronous` or the engine thread), and it calls them one at a time, synchronously, and in order. But I don't see that in the [documentation for loggers](https://docs.microsoft.com/en-us/visualstudio/msbuild/build-loggers). There's a lot of talk of multi-**process** models but none about threading in the loggers themselves.