update feature flag documentation for 'hidden_in_prod'

refs CNVS-16352

Change-Id: Icfbfce03c48a48fbe07f531be90406fe278f2a8d
Reviewed-on: https://gerrit.instructure.com/43389
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
Product-Review: Jon Willesen <jonw@instructure.com>
QA-Review: Jon Willesen <jonw@instructure.com>