Followup to #20583 (observed keyword for Groupby)
1. Change Grouping.__init__ default observed to False: https://github.com/pandas-dev/pandas/pull/20583#discussion_r185209589
2. whatsnew description on unobserved, cartesian product: https://github.com/pandas-dev/pandas/pull/20583#discussion_r185208219
3. Simplify implementation? https://github.com/pandas-dev/pandas/pull/20583#discussion_r185203879
4. rst formatting: https://github.com/pandas-dev/pandas/pull/20583/files/bdf7525812ca670f9406ab8df333030d36d30947#diff-03b380f521c43cf003207b0711bac67f
5. stale comment? https://github.com/pandas-dev/pandas/pull/20583#discussion_r185209781