api: restore docs for creating a submission
api: restore docs for creating a submission

fixes CNVS-10241

test plan:
- generate the api docs and go to /doc/api/submissions.html
- there should be docs for creatint a submission

Change-Id: I734127c194a10c2e33af3923e541c994f328216c
Reviewed-on: https://gerrit.instructure.com/28017
Reviewed-by: Cameron Matheson <1d572acbfa68c7c6e541c7b840d6b622e5c0dc91@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
QA-Review: Caleb Guanzon <54fa1f30d451e4f3c341a8f3c47c9215fd0be9c6@instructure.com>
Product-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>

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