add documentation about associated user

refs CNVS-12065

test plan
 - docs should generate
 - specs should pass

Change-Id: Id08342e484259e9c38de70106572209d2a9219b2
Reviewed-on: https://gerrit.instructure.com/33243
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>