docs: recommend .end instead of .send when no body is being sent (#33611)

Fixes #33536

I was thinking to make `body` optional here instead:

2c6bd1ed1b/packages/next/shared/lib/utils.ts (L262)

But that might result in more errors when not setting the status code correctly. So it's the easiest for us to just recommend `.end()` which `.send()` uses when there is no argument.

c71465d87a/packages/next/server/api-utils.ts (L269)

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `yarn lint`

errors/invalid-api-status-body.md

@@ -22,7 +22,7 @@ After
 
 ```js
 export default function handler(req, res) {
-  res.status(204).send()
+  res.status(204).end()
 }
 ```