closes vercel#55050, add error doc

errors/reserved-port.mdx

@@ -0,0 +1,27 @@
+---
+title: Reserved Port
+---
+
+## Why This Error Occurred
+
+Server was started on a reserved port. For example, `4045` is reserved for the Network Paging Protocol (npp).
+
+```
+next start -p 4045
+```
+
+or
+
+```
+next dev -p 4045
+```
+
+Starting the server on a reserved port will result in an error.
+
+## Possible Ways to Fix It
+
+Visit the [port blocking](https://fetch.spec.whatwg.org/#port-blocking) section of WHATWG's fetch spec and ensure your port is not on the known reserved port list.
+
+## Useful Links
+
+- https://fetch.spec.whatwg.org/#port-blocking

test/integration/cli/test/index.test.js

@@ -533,7 +533,10 @@ describe('CLI Usage', () => {
     })
 
     test('-p reserved', async () => {
-      const { stderr, stdout } = await runAndCaptureOutput({ port: 1 })
+      const TCP_MUX_PORT = 1
+      const { stderr, stdout } = await runAndCaptureOutput({
+        port: TCP_MUX_PORT,
+      })
 
       expect(stdout).toMatch('')
       expect(stderr).toMatch("Bad port: '1' is reserved for tcpmux")