I added a couple of lines to the 3.2, 3.3 print doc in #11633. When a change is made here, they could be updated to something like:
"Output buffering is normally determined by *file*. Use flush=True to force immediate output to a device such as a screen."

Unless there is a good reason, I prefer, unlike the other params, *not* allowing flush=None as a synonym for flush=<the default, ie False>.