What is the purpose of appending this? (or any additional text to the error log):

View in rendered output by enabling the "displayErrorDetails" setting.

to the error_log() in the first place?

@l0gicgate This confuses me as well. What are the arguments for appending anything additional here? https://github.com/slimphp/Slim/blob/4.x/Slim/Handlers/ErrorHandler.php#L271

If there aren't any compelling arguments then I propose the line should be removed.

@RyanNerd so if someone was to look at the logs and see that there are no error details (stack trace). It would indicate to them that by turning on displayErrorDetails on ErrorHandler they could see the stack trace via a browser/http client by toggling that on. It's really just a "tip"

This is marked as an easy pick but changing the message to something else is very subjective. Perhaps something like:

To see error details set displayErrorDetails to true in the ErrorHandler.

Perhaps, something like:
Tips: To display error details in HTTP response set "displayErrorDetails" to true in the ErrorHandler constructor.

And you can add this line only if "$displayErrorDetails" is equal to false.