re-works the progression of the concepts to have a more linear narrative;
improves the wording and fixes a typo.
Discussing with @ogrisel we thought that this one can be updated in FUN even if new users start having a slightly different version, but I would rather have the @glemaitre and @lesteve opinions.
Fixes #752. This PR:
Discussing with @ogrisel we thought that this one can be updated in FUN even if new users start having a slightly different version, but I would rather have the @glemaitre and @lesteve opinions.