copy discussion 'create' API docs to 'update'

Since YARD doesn't know about arguments in other methods, we need
to copy them over. Provides access for autogenerated Canvas API
tools such as Pandarus.

Fixes SIS-551

Change-Id: I31080057c4c955ed08e94523e2ff0f5461114212
Reviewed-on: https://gerrit.instructure.com/40989
Reviewed-by: Brad Horrocks <bhorrocks@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Tyler Pickett <tpickett+gerrit@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>