Adds PHPDoc to Logger::log

> The 8 standard levels must be implemented as strings to satisfy the contract, but others are allowed so as long as they are defined

Yeah. I think you're right.

> MUST throw a Psr\Log\InvalidArgumentException

Hmm, I see I did not comply with that. Did I miss anything else?

> Not sure I understand so to clarify

I meant if you could kindly add a usage example in the PHPDoc, similar to the way we have in the PHPDoc of the `Log::add` method. But that usage example should differ in that `$message` string variables need to have the braces around.