doc: Mention bitcoin wrapper executable in documentation

doc/zmq.md

@@ -87,6 +87,8 @@ For instance:
                -zmqpubrawtx=ipc:///tmp/bitcoind.tx.raw \
                -zmqpubhashtxhwm=10000
 
+`bitcoin node` or `bitcoin gui` can also be substituted for `bitcoind`.
+
 Notification types correspond to message topics (details in next section). For instance,
 for the notification `-zmqpubhashtx` the topic is `hashtx`. These options can also be
 provided in bitcoin.conf.