add argument to sections api documentation

adds documentation for the
"restrict_enrollments_to_section_dates" argument

refs #CNVS-17652

Change-Id: I51e2cc006b2434f8bda0d03016005f6db0c4469a
Reviewed-on: https://gerrit.instructure.com/46304
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>