add missing fields to Assignments API doc
add missing fields to Assignments API doc

important_dates, muted, anonymize_students, require_lockdown_browser

fixes EVAL-2211
flag=none

Test plan:
- Generate docs
- Confirm fields are included in doc/api/assignments.html

Change-Id: I0fd2f7d31fdfea84493e13acad4f02cf4bb748fa
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/287611
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Eduardo Escobar <7e82d09bba767b0dd1222915e3d92141089e94a8@instructure.com>
Reviewed-by: Kai Bjorkman <4693b3d9e71784d629732770ab5e83ed8fbdb83e@instructure.com>
QA-Review: Dustin Cowles <25afa0fec7fcf03720748195b533ef56026fd0cf@instructure.com>
Product-Review: Jody Sailor

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms