Improve readability of DEHB examples and docstrings

It will be useful to have more elaborate _filler_ text in the notebooks serving as [examples](https://github.com/automl/DEHB/tree/master/examples) for using DEHB. 

Also, it will be useful to have more docstrings that help explain DEHB function arguments. The example notebooks should also highlight how information on the arguments can be retrieved, for example as `?dehb.run`.