allow API Doc args to use "Required"

This is a step toward making auto-generated code from docs not
trip up over every new argument that has been added (developers
often forget to mark new arguments as "Optional" which makes them
default to required. We want new arguments to default to
optional.)

Fixes SIS-399

Change-Id: I8cfa5516a6fdb1b0461bca3fa34fd80a763686ba
Reviewed-on: https://gerrit.instructure.com/38574
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ben Young <byoung@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>