Document how to debug connection issues

[RoSk0]

I've spent enormous amount of time trying to debug my connection issue. The main reason was lack of proper documentation on php.net which I'm trying to resolve here.

There is a TODO in the proposed change - the reason for it is that I'm not sure where that magical value 7 is coming from and what level it relates to. I've scanned the php-src code base, but found nothing useful. That TODO comment represents my findings.

[Girgias]

@mcmic could you review this when you have time?

[RoSk0]

Debug output goes to the STDOUT/STDERR. Not sure which as I was testing in the container environment- when script was called from CLI, the output was visible straight away, when called via output was on on php-fpm container log.