Initial documentation for the new ZMQ_PROTOCOL socket option

doc/zmq_getsockopt.txt

@@ -455,6 +455,20 @@ Option value unit:: -1,>0
 Default value:: -1 (leave to OS default)
 Applicable socket types:: all, when using TCP transports.
 
+ZMQ_PROTOCOL: Wire protocol version in use
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Retrieve the wire protocol version used by this socket (pipe actually). A value
+of `1` designates backwards compatibility with existing 2.x pub-sub components
+in an existing topology. A value of `0` means subscription forwarding is
+enabled for pub-sub pairs.
+
+[horizontal]
+Option value type:: int
+Option value unit:: >0
+Default value:: 0 (with subscription forwarding)
+Applicable socket types:: all (only respected by ZMQ_PUB and ZMQ_SUB)
+
 RETURN VALUE
 ------------
 The _zmq_getsockopt()_ function shall return zero if successful. Otherwise it

doc/zmq_setsockopt.txt

@@ -431,6 +431,20 @@ Default value:: no filters (allow from all)
 Applicable socket types:: all listening sockets, when using TCP transports.
 
 
+ZMQ_PROTOCOL: Specify the wire protocol version to use
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ZeroMQ 3.x extended the pub-sub protocol to support subscription forwarding,
+which isn't compatible with 2.x pub-sub semantics. The 'ZMQ_PROTOCOL' option
+allows for specifying the protocol version to use. A value of `1` designates
+backwards compatibility with existing 2.x pub-sub components in an existing
+topology.
+
+[horizontal]
+Option value type:: int
+Option value unit:: >0
+Default value:: 0 (with subscription forwarding)
+Applicable socket types:: all (only respected by ZMQ_PUB and ZMQ_SUB)
+
 RETURN VALUE
 ------------
 The _zmq_setsockopt()_ function shall return zero if successful. Otherwise it