Today the docs use `logger.org.elasticsearch.transport: TRACE` as the
example for adjusting the logger config. This is a dangerous thing to
suggest since that's one of the most verbose loggers we have. An
accidental copy-and-paste of this example into a busy cluster can
cause damage.
This commit suggests `logger.org.elasticsearch.discovery: DEBUG`
instead, which is much more benign.
It also corrects the order of the levels and notes that `DEBUG` and
`TRACE` are only for expert use.
b706e0dd69