Refer to zmq_socket because there are more thread-safe sockets than just client/server

doc/zmq_getsockopt.txt

@@ -786,8 +786,8 @@ Applicable socket types:: all, when using TCP transports.
 ZMQ_THREAD_SAFE: Retrieve socket thread safety
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 The 'ZMQ_THREAD_SAFE' option shall retrieve a boolean value indicating whether
-or not the socket is threadsafe. Currently 'ZMQ_CLIENT' and 'ZMQ_SERVER' sockets
-are threadsafe.
+or not the socket is threadsafe. See linkzmq:zmq_socket[3] for which sockets are
+thread-safe.
 
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 Option value type:: boolean