Hx tracking -- update comments etc. to reflect current terminology and implementation

[rod-glover]

I noticed that the comments in pycds/orm/trigger_functions/version_7ab87f8fbcf4.py used outdated terminology and had some slightly outdated explanations. Fix these.

Also the variable names in the database trigger functions reflect this outdatedness. It would be best to rename them, but strictly speaking that is a migration (to replace the existing tf's with the revised ones). I will leave that to someone else's discretion, but offer suggested renamings in comments.