Followup to #20583 (observed keyword for Groupby)

[TomAugspurger]

1. Change Grouping.init default observed to False: API: categorical grouping will no longer return the cartesian product #20583 (comment)
2. whatsnew description on unobserved, cartesian product: API: categorical grouping will no longer return the cartesian product #20583 (comment)
3. Simplify implementation? API: categorical grouping will no longer return the cartesian product #20583 (comment)
4. rst formatting: https://github.com/pandas-dev/pandas/pull/20583/files/bdf7525812ca670f9406ab8df333030d36d30947#diff-03b380f521c43cf003207b0711bac67f
5. stale comment? API: categorical grouping will no longer return the cartesian product #20583 (comment)