Point out that the setdefault defaults the value to None. Inspired by

Michael Chermside's thinking about patch #748126 (the chief upshot of
which thinking was "reject it!").

Doc/lib/libstdtypes.tex

@@ -1368,7 +1368,7 @@ \subsection{Mapping Types \label{typesmapping}}
 
 \item[(5)] \function{setdefault()} is like \function{get()}, except
 that if \var{k} is missing, \var{x} is both returned and inserted into
-the dictionary as the value of \var{k}.
+the dictionary as the value of \var{k}. \var{x} defaults to \var{None}.
 
 \item[(6)] \function{popitem()} is useful to destructively iterate
 over a dictionary, as often used in set algorithms.