add created_at to enrollment api object

also updated enrollment api documentation

closes CNVS-10391

test plan
 - hit an enrollment endpoint 
   /api/v1/courses/:course_id/enrollments
 - api json should contain created_at
 - run ram doc:api
 - /doc/api/enrollments.html should render

Change-Id: If1b252a82d2e14bed7222b44a6e5e5e5785b738a
Reviewed-on: https://gerrit.instructure.com/28420
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>