Document that log() for ELEMENTS in streams requires a DEBUG logging level

[rucek]

After just following the docs for using log() the logging "doesn't work", i.e. the logged elements are not visible in the logs. Thisi is because the logging level nedds to be set to DEBUG with akka.loglevel = "DEBUG", which the docs fail to mention.

[ktoso]

I disagree, that's not what we'd want to document: I guess we can add that to the docs, though the actual intended use is different and documented already: The docs show how to change the log level of "per element:

.withAttributes(Attributes.logLevels(onElement = Logging.WarningLevel))

if that's unclear that should be clarified, perhaps along with the fact that by default the elements are logged on debug?

[rucek]

You're right, I missed it somehow. Then I'd just add a note to clarify that the default level is debug. If you're OK with it, a PR will be there shortly.