Fix wording and clarify that the IDNA codec operates on full domain names.
Fix wording and clarify that the IDNA codec operates on full domain names.

Before reading the code to check, I wasn't sure if it operated on
full domain names or just individual labels.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator