Fix typo

These constants are key usage numbers, not keys themselves.

Change-Id: I63be6941ad33cc41c31734c0dee447ae2accad49
Use less-deceptive art for AUTH

Don't imply that the MIC is a multiple of four octets in length,
and explicitly disclaim alignment of the following payload.

Change-Id: I8e4ac3ed0e2595270df8d00ce471d318d2d009c8