Fix CELERYD_HIJACK_ROOT_LOGGER description

`CELERYD_HIJACK_ROOT_LOGGER` doesn't actually reset e.g. logger
levels and filters, or handlers and formatters attached to loggers
other than the root logger. Just removes any handlers already
attached to the root logger. This says so more clearly.
Merge pull request #1197 from gthb/patch-2

Fix CELERYD_HIJACK_ROOT_LOGGER description