Add docs for Process::getIterator()

[javiereguiluz]

Should we add docs for this method? We recently improved it adding config flags to it: symfony/symfony#18513

[javiereguiluz]

I'm trying to document this but I'm having problems deciding the purpose of this getIterator() method. Does it:

1. Replace/improve the existing way to get the normal output:

$process = new Process('ls -lsa');
$process->start();

foreach ($process as $type => $data) {
    // ...
}

2. Replace/improve the existing way the output in real time:

$process = new Process('ls -lsa');
$process->run(function ($type, $buffer) {
    // ...
}

3. Is it something completely different?

---

Let's ping some Process experts: @romainneutron @nicolas-grekas

[javiereguiluz]

On Symfony Slack, @stof answered me:

Process::getIterator is how foreach ($process as $type => $data) is implemented (through IteratorAggregate) but calling the method explicitly allows to use the optional arguments to tweak the behavior