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?
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:
If it is not officially documented I would guess the hook is there until it disappears :-)