Update HACKING.rst per recent changes

Added "is not" usage with examples
* https://review.openstack.org/#/c/20865/

Fixed "not in" usage description
* https://review.openstack.org/#/c/20875/

Fixed some "not X in Y" corner cases.

Change-Id: I7534ef73e6fd525fd8f4bee594a4b37524699c08
Merge "Update HACKING.rst per recent changes"