Thanks for the reply, Collin. Perhaps something like:
Note: The value stored in returncode may be out-of-date. Use poll() to reliably find the current return code.

Alternatively, would it be possible to leave it out completely [and move the explanation of what return codes mean up into the description of poll()]? I can´t imagine many situations where it is a good idea to check returncode directly, and as I understand it the module documentation only covers variables that are useful to the outside world.