docs: add EventSource server example (#5321)

Signed-off-by: Will-thom <116388885+Will-thom@users.noreply.github.com>

Author: Will-thom

docs/docs/api/EventSource.md

@@ -7,7 +7,7 @@ for [Server-Sent Events](https://developer.mozilla.org/en-US/docs/Web/API/Server
 
 ## Instantiating EventSource
 
-Undici exports a EventSource class. You can instantiate the EventSource as
+Undici exports an EventSource class. You can instantiate the EventSource as
 follows:
 
 ```mjs
@@ -19,9 +19,57 @@ eventSource.onmessage = (event) => {
 }
 ```
 
+## Receiving events from a server
+
+EventSource connects to an HTTP endpoint that responds with a `text/event-stream`
+content type. The connection stays open and receives events as the server writes
+them.
+
+```mjs
+import { createServer } from 'node:http'
+import { EventSource } from 'undici'
+
+const server = createServer((request, response) => {
+  response.writeHead(200, {
+    'content-type': 'text/event-stream',
+    'cache-control': 'no-cache',
+    connection: 'keep-alive'
+  })
+
+  response.write('event: ping\n')
+  response.write('data: connected\n\n')
+
+  const interval = setInterval(() => {
+    response.write(`data: ${Date.now()}\n\n`)
+  }, 1000)
+
+  request.on('close', () => clearInterval(interval))
+})
+
+server.listen(3000, () => {
+  const eventSource = new EventSource('http://localhost:3000')
+
+  eventSource.addEventListener('ping', (event) => {
+    console.log('ping:', event.data)
+  })
+
+  eventSource.onmessage = (event) => {
+    console.log('message:', event.data)
+  }
+
+  eventSource.onerror = () => {
+    eventSource.close()
+    server.close()
+  }
+})
+```
+
+The `message` event receives events without an explicit `event:` field. Use
+`addEventListener()` to subscribe to named events.
+
 ## Using a custom Dispatcher
 
-undici allows you to set your own Dispatcher in the EventSource constructor.
+Undici allows you to set your own Dispatcher in the EventSource constructor.
 
 An example which allows you to modify the request headers is:
 
@@ -38,7 +86,6 @@ class CustomHeaderAgent extends Agent {
 const eventSource = new EventSource('http://localhost:3000', {
   dispatcher: new CustomHeaderAgent()
 })
-
 ```
 
 More information about the EventSource API can be found on