docs:  clarify behavior when no recv callback is registered
docs:  clarify behavior when no recv callback is registered

caidongyun/pyzmq,yyt030/pyzmq,caidongyun/pyzmq,yyt030/pyzmq,Mustard-Systems-Ltd/pyzmq,swn1/pyzmq,dash-dash/pyzmq,dash-dash/pyzmq,ArvinPan/pyzmq,ArvinPan/pyzmq,yyt030/pyzmq,dash-dash/pyzmq,Mustard-Systems-Ltd/pyzmq,ArvinPan/pyzmq,swn1/pyzmq,swn1/pyzmq,Mustard-Systems-Ltd/pyzmq,caidongyun/pyzmq