This is different than the post hook, which is officially documented. The only real reference to the postSave hook I can find is CRM-7850, so I'm just wondering if it's supported or if it might disappear in future?
2 Answers
Reading the code and that issue, the hook exists and someone just forgot to add it to the wiki. Here, I've just added it:
http://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_postSave_table_name
If it is not officially documented I would guess the hook is there until it disappears :-)