clarify how to include groups in collaborations
clarify how to include groups in collaborations

why:
* some confusion over the format of the array of group ids in a
collaboration deep linking response
* clarify that the groups should be identified by LTI id, not by
Canvas id

refs INTEROP-6876, INTEROP-6601
flag=none

test plan:
* run `rake doc:api` in or out of docker, depending on your setup
* the changes to the .md file should be reflected in the .html file, at
https://canvas.docker/doc/api/file.collaborations_placement.html

Change-Id: I3bcfca05659a65b42e717888919c5a04ca298450
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/267417
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Mysti Lilla <431d062aff1e8f25416b3a7b732fe6effdb03f9a@instructure.com>
QA-Review: Xander Moffatt <27f70fe194da423604e6e0a63a3de19814736fa9@instructure.com>
Product-Review: Xander Moffatt <27f70fe194da423604e6e0a63a3de19814736fa9@instructure.com>

sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms