Document how to use Auto-WEKA's/SMAC's intensify

[mfeurer]

We currently only document how to use CV etc., but not how to use the mode from the Auto-WEKA. This should be documented in the respective example: https://automl.github.io/auto-sklearn/master/examples/40_advanced/example_resampling.html#cross-validation

[mfeurer]

And also how to pass the group argument to the splitter function when using grouped CV