Sprinkle the MOOC with comments on what the evaluation means

As mentioned by @ogrisel : we could add a small sentence in many places in the MOOC where we do model evaluation commenting on what the evaluation means: the choice of evaluation method, the unit, ... going back the real life.

No specific files or notebooks are named. Search the MOOC notebooks for model-evaluation sections, then review how evaluation methods, units, and real-world interpretation are currently explained. Done means the relevant sections consistently include concise context about what their evaluation results mean.

Written by the indexing model from the issue text.