stdin is mentioned in the documentation of the -u option only due to weird internal buffering in Python 2, because user can expect that -u disables it. It is documented what methods use internal buffering and how get rid of it. No other buffering is mentioned.

This no longer actual in Python 3. I think there is no need to mention stdin in the context of the -u option at all. -u doesn't affect stdin buffering, whatever that would mean. Period.

Alternatively you can include a lecture about different kinds of buffering and how -u doesn't affect them.