This new feature comes from
MAPREDUCE-6052, some documentation requirements from Vinod below:
Document the new config in mapred-default.xml
Mention in that documentation that if no-scheme is given in the path, it defaults to a log4j file on the local FS.
Modify the documentation of log-level configs to say that if you override to have your own log4j.properties file, the log-level configs may not work.