doc: document that addMembership must be called once in a cluster

Fixes: nodejs#12572
Refs: nodejs#16240

Author: jasnell

doc/api/dgram.md

@@ -95,6 +95,24 @@ Tells the kernel to join a multicast group at the given `multicastAddress` and
 one interface and will add membership to it. To add membership to every
 available interface, call `addMembership` multiple times, once per interface.
 
+When sharing a UDP socket across multiple `cluster` workers, the
+`socket.addMembership()` function must only be called *only once* or an
+`EADDRINUSE` error will occur:
+
+```js
+const cluster = require('cluster');
+const dgram = require('dgram');
+ if (cluster.isMaster) {
+   cluster.fork(); // Works ok
+   cluster.fork(); // Fails with EADDRINUSE
+} else {
+  const s = dgram.createSocket('udp4');
+  s.bind(1234, () => {
+    s.addMembership('224.0.0.114');
+  });
+}
+```
+
 ### socket.address()
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