doc: clarify subprocess.stdout/in/err property

[cola119]

Fix #43905

subprocess.stdout/in/err property can be undefined when EMFILE or ENFILE error happend.

node/test/sequential/test-child-process-emfile.js

Lines 58 to 65 in 389b7e1

	// Should emit an error, not throw.
	const proc = child_process.spawn(process.execPath, ['-e', '0']);
	// Verify that stdio is not setup on EMFILE or ENFILE.
	assert.strictEqual(proc.stdin, undefined);
	assert.strictEqual(proc.stdout, undefined);
	assert.strictEqual(proc.stderr, undefined);
	assert.strictEqual(proc.stdio, undefined);

And they can be null when the child process couldn't be successfully spawned.

node/lib/internal/child_process.js

Lines 455 to 460 in 389b7e1

	this.stdin = stdio.length >= 1 && stdio[0].socket !== undefined ?
	stdio[0].socket : null;
	this.stdout = stdio.length >= 2 && stdio[1].socket !== undefined ?
	stdio[1].socket : null;
	this.stderr = stdio.length >= 3 && stdio[2].socket !== undefined ?
	stdio[2].socket : null;

[aduh95]

Should we also update these lines?

node/doc/api/child_process.md

Line 1673 in 72fd06f

* {stream.Readable}

node/doc/api/child_process.md

Line 1692 in 72fd06f

* {stream.Writable}

node/doc/api/child_process.md

Line 1758 in 72fd06f

* {stream.Readable}

[cola119]

@aduh95 You mean this part?

node/doc/api/child_process.md

Lines 1762 to 1763 in 72fd06f

	If the child was spawned with `stdio[1]` set to anything other than `'pipe'`,
	then this will be `null`.

[aduh95]

@aduh95 You mean this part?

Hum no, I meant the three lines I've linked to (instead of {stream.Readable}, it should be {stream.Readable|null|undefined}, etc.).

[cola119]

Thanks for the review @aduh95. PTAL.

[nodejs-github-bot]

Landed in a324ea0