Adds PHPDoc to Logger::log

@enov Yeah, it's a bit confusing the definition they provide for the log levels as string, but then mixed in the interface:

>A ninth method, log, accepts a log level as first argument. Calling this method with one of the log level constants MUST have the same result as calling the level-specific method. Calling this method with a level not defined by this specification MUST throw a Psr\Log\InvalidArgumentException if the implementation does not know about the level. Users SHOULD NOT use a custom level without knowing for sure the current implementation supports it.

It looks like that would open the possibility for an implementation to add more levels other than the ones provided, which aren't necessarily required to be strings as per the spec. The 8 standard levels must be implemented as strings to satisfy the contract, but others are allowed so as long as they are defined?

> Don't forget the braces in the message.

Not sure I understand so to clarify, do you mean in the commit message to write as Logger::log()  rather Logger::log, or something else?