Document running inner class tests in xUnit console

While I was trying to run tests in an inner class using xunit console, I couldn't figure it out until I see this issue #913. In 2.1, the -class+ switch is already added. And the working way of specifying an inner class is using namespace.class+innerclass rather than using the . notation used in C#. There is no documentation on either of this.

Steps: xunit.console.exe -?

Expect: Documentation should mention inner class using -class namespace.class+innerclass and -class+. Also for methods -method namespace.class+innerclass.method