update API doc required params - 12

- User observees
- Users

Test plan:
- rake doc:api
- Verify the above doc pages for parameter accuracy

Refs SIS-403

Change-Id: If7b362adf1aa385483d362557e88fabafbb39426
Reviewed-on: https://gerrit.instructure.com/39370
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Duane Johnson <duane@instructure.com>
Product-Review: Duane Johnson <duane@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>