Document node reuse

I searched docs.microsoft.com for details about node reuse so I could send them to a person who was asking. But there was basically nothing, and a search for [msbuild node reuse] wasn't super helpful, either.

There's an announcement blog post: https://blogs.msdn.microsoft.com/msbuild/2007/04/16/node-reuse-in-multiproc-msbuild/

But since it's important to tell people with CI servers and so on to disable node reuse, we should have clear documentation in the usual place.