C# documentation improvements

Last week I reached out to some repository owners who enabled C# support about some feedback.

The overall response was really positive and most didn't run into any major problems. One of the common topics that came up though was that there's no clear guidance on how to run NUnit/xunit tests.

I added a section to the docs about that and also clarified some minor points in the other sections that caused confusion.

/cc @Joshua-Anderson @nterry