How about following "The Python interpreter has a number of functions and types built into it that are always available. They are listed here in alphabetical order." in
https://docs.python.org/3/library/functions.html
with "Here and elsewhere in these docs, entries for functions (including methods) that always return None usually omit 'Return None' and just say what the function does."

Barry: The PEP 8 'return None' recommendation could be added to the Reference entry for 'return'.  But I think this should be a separate issue as 1) it is about coding rather than documentation and 2) there is the possible objection that choosing completely explicit 'return None' versus half explicit, half implicit 'return' and the latter versus completely implicit <nothing at end> should be left to the style PEP.