Improve documentation for dynamic classes.

An attempt to improve the clarity of the documentation for using verifying doubles with dynamic classes.
I think overriding `method_defined?` is a bad idea.  If `MyKlass.method_defined?(:foo)` returns true, I would expect `MyKlass.instance_method(:foo)` to return an `UnboundMethod` instance of the `foo` method...but that's not going to happen here.  In addition, which messages a dynamic class responds to is often based on instance state that isn't available at the class level, so overriding `method_defined?` isn't a general solution.