doc: recommend events.once to manage 'close' event

`events.once` is a great way to manage the `close` / `error` / `exit`
events of a child process. It creates a Promise that is fulfilled when
the EventEmitter emits the given event or that is rejected if the
EventEmitter emits 'error' while waiting.

I think a lot of people wrongly think that managing a spawned child
process requires writing boilerplate handlers for `close` and `error`,
and that there's no `promisify` solution for spawned child processes. In
fact, `events.once` is that solution. The docs should explicitly
recommend it in examples.

Author: dfabulich

doc/api/child_process.md

@@ -29,6 +29,7 @@ ls.on('close', (code) => {
 
 ```mjs
 import { spawn } from 'node:child_process';
+import { once } from 'node:events';
 const ls = spawn('ls', ['-lh', '/usr']);
 
 ls.stdout.on('data', (data) => {
@@ -39,9 +40,8 @@ ls.stderr.on('data', (data) => {
   console.error(`stderr: ${data}`);
 });
 
-ls.on('close', (code) => {
-  console.log(`child process exited with code ${code}`);
-});
+const [code] = await once(ls, 'close');
+console.log(`child process exited with code ${code}`);
 ```
 
 By default, pipes for `stdin`, `stdout`, and `stderr` are established between
@@ -772,6 +772,7 @@ ls.on('close', (code) => {
 
 ```mjs
 import { spawn } from 'node:child_process';
+import { once } from 'node:events';
 const ls = spawn('ls', ['-lh', '/usr']);
 
 ls.stdout.on('data', (data) => {
@@ -782,9 +783,8 @@ ls.stderr.on('data', (data) => {
   console.error(`stderr: ${data}`);
 });
 
-ls.on('close', (code) => {
-  console.log(`child process exited with code ${code}`);
-});
+const [code] = await once(ls, 'close');
+console.log(`child process exited with code ${code}`);
 ```
 
 Example: A very elaborate way to run `ps ax | grep ssh`
@@ -1483,6 +1483,7 @@ ls.on('exit', (code) => {
 
 ```mjs
 import { spawn } from 'node:child_process';
+import { once } from 'node:events';
 const ls = spawn('ls', ['-lh', '/usr']);
 
 ls.stdout.on('data', (data) => {
@@ -1496,6 +1497,9 @@ ls.on('close', (code) => {
 ls.on('exit', (code) => {
   console.log(`child process exited with code ${code}`);
 });
+
+const [code] = await once(ls, 'close');
+console.log(`child process close all stdio with code ${code}`);
 ```
 
 ### Event: `'disconnect'`