Update examples for i18n

The examples for logging i18n messages are out of date based on
changes in openstack/common/gettextutils.py and oslo.i18n.
Aside from updating the examples, also introduce a link to the
location where more guidelines will be found in the future.

Change-Id: Ic9ee36b223c522012fc4443597f4568dad9bcd67
Merge "Update examples for i18n"