support special api ids for terms

also, document all special ids

Change-Id: Ife484c02343f8b109eecb3e3b2b4a408f48895b3
Reviewed-on: https://gerrit.instructure.com/30008
QA-Review: August Thornton <august@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>