Documentation: Clarify mapping from high-level agent properties to experiments and environments

Hello Ian and others!

I'm having a look at `bsuite` after Ian Osband's talk at the Simons Institute Deep RL workshop. After spending a few minutes browsing the documentation and source code here on GitHub I had a suggestion for improving the documentation.

My first question when browsing this project is "*The radar plot on the readme is lovely, I wonder what experiments contribute to a good ____________ score*". Where the blank is e.g. 'generalization'.

After browsing the source code for a few minutes this isn't immediately obvious. I can see a little bit of information regarding this at the example [colab notebook](https://colab.research.google.com/drive/1RYWJaMEHVeN8yI83QtL35GOSFQBRgLaX#scrollTo=zCNVq9M0IEpT). It would be nice to promote this mapping to a 'first class' member of the documentation somewhere :)