add API docs for integration_id/data

API docs are missing integration_id/integration_data for
assignment 'update' API call.

Test Plan:
- Update Assignment API docs should include integration_id/data

Fixes SIS-374

Change-Id: I088796e2aac274ba90a048c55f9b53dcceb63be4
Reviewed-on: https://gerrit.instructure.com/37949
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Tyler Pickett <tpickett+gerrit@instructure.com>
Product-Review: Tyler Pickett <tpickett+gerrit@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>