Clarify integration_data field
Clarify integration_data field

closes CNVS-38316

test plan:
  - generate the latest docs:
    > docker-compose run --rm web rake doc:api
  - start up canvas:
    > docker-compose up
  - load assignments doc at http://canvas.docker/doc/api/assignments.html
  - search for "assignment[integration_data]"
  - confirm that the two instances of the above mention SIS integration

Change-Id: Ia8ff58ca8eaa988b4129cfb616d418001eee8570
Reviewed-on: https://gerrit.instructure.com/125213
Reviewed-by: Matt Berns <e53c8988cd428dde240324ce2797c8eb7073560f@instructure.com>
Tested-by: Jenkins
QA-Review: Andrew Porter <747194079243b0beca9081f8cd559c020b06a314@instructure.com>
Product-Review: Oxana

djbender/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,venturehive/canvas-lms,instructure/canvas-lms