Boby,
Please refer to Collect Logs & Diagnostic Information(UI) for a futher explanation.
In addition I do agree that the https://docs.couchbase.com/server/6.5/eventing/eventing-debugging-and-diagnosability.html#logging-functions is a bit lacking on the subject of log files. This deficiency has been rectified and is pending final review for an update in the next week or two.
Thanks
Jon