Update HACKING.rst per recent changes

Added "is not" usage with examples
Fixed "not in" usage description

https://review.openstack.org/#/c/20576/
and
https://review.openstack.org/#/c/20879/

Fixes a previous "not X in Y" grep corner case

Change-Id: I92ae642887de734a8562a528003d8092f5ff27d4
Merge "Update HACKING.rst per recent changes"