My question was answered on the Jetty mailing list by Joakim Erdfelt:
You are looking at the old Jetty 6.x docs at docs.codehaus.org.
DEBUG logging is just a logging level determined by the logging
implementation you choose to use.
-D{classref}.LEVEL={level}
Where {classref} is the class reference you want to set the level on,
and all sub-class refs. and {level} is one of the values ALL, DEBUG,
INFO, WARN
Example:
-Dorg.eclipse.jetty.LEVEL=INFO - this will enable INFO level logging for all jetty packages / classes.
-Dorg.eclipse.jetty.io.LEVEL=DEBUG - this will enable DEBUG level logging for IO classes only
-Dorg.eclipse.jetty.servlet.LEVEL=ALL - this will enable ALL logging (trace events, internally ignored exceptions, etc..) for servlet
packages.
-Dorg.eclipse.jetty.util.thread.QueuedThreadPool.LEVEL=ALL - this will enable level ALL+ on the specific class only.