doc: clarify when 'listening' event is emitted

Clarify that the `'listening'` event is emitted when the
underlying OS socket is created

This occurs when either `socket.send()` or `socket.bind()`
are first invoked for the respective socket.

Fixes: #23952

doc/api/dgram.md

@@ -64,8 +64,8 @@ function is passed a single `Error` object.
 added: v0.1.99
 -->
 
-The `'listening'` event is emitted whenever a socket begins listening for
-datagram messages. This occurs as soon as UDP sockets are created.
+The `'listening'` event is emitted on the first invocation of either `socket.send()`
+or `socket.bind()` for the respective socket.
 
 ### Event: 'message'
 <!-- YAML