fix yardoc warnings

test plan:
- make sure `rake doc:api` doesn't generate warnings

Change-Id: Icd76484ef72923f2de1902b8b05f479bdfc02f57
Reviewed-on: https://gerrit.instructure.com/33459
Reviewed-by: Rob Orton <rob@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>