api doc: add missing allowed params to discussion create/update
api doc: add missing allowed params to discussion create/update

fixes CNVS-22245

test plan:
- render the api documentation, and look at the discussion topic section
- verify that in the update method, 'pinned' is listed as a param
- verify that in the create method, 'pinned', 'allow_rating',
  'only_graders_can_rate', and 'sort_by_rating' are listed
- verify that all the newly listed params are functional

Change-Id: I0f055d975c4a0be1a4edce735c70d8b4e176f6a2
Reviewed-on: https://gerrit.instructure.com/59614
Tested-by: Jenkins
Reviewed-by: Cameron Sutter <853bcc0cb8f5834b7805e8eb9a762054be43c731@instructure.com>
QA-Review: Adam Stone <a0b2b61e2edcf99aa16080c2254a6ae6923d1fac@instructure.com>
Product-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>

leftplusrightllc/canvas-lms,antoniuslin/canvas-lms,whalejasmine/canvas-lms,sfu/canvas-lms,greyhwndz/canvas-lms,HotChalk/canvas-lms,matematikk-mooc/canvas-lms,jay3126/canvas-lms,fronteerio/canvas-lms,roxolan/canvas-lms,antoniuslin/canvas-lms,AndranikMarkosyan/lms,HotChalk/canvas-lms,antoniuslin/canvas-lms,fronteerio/canvas-lms,HotChalk/canvas-lms,AndranikMarkosyan/lms,whalejasmine/canvas-lms,snehakachroo1/LSI_replica,jay3126/canvas-lms,utkarshx/canvas-lms,grahamb/canvas-lms,instructure/canvas-lms,sdeleon28/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,narigone/canvas,venturehive/canvas-lms,jay3126/canvas-lms,redconfetti/canvas-lms,snehakachroo1/LSI_replica,efrj/canvas-lms,leftplusrightllc/canvas-lms,roxolan/canvas-lms,AndranikMarkosyan/lms,instructure/canvas-lms,greyhwndz/canvas-lms,utkarshx/canvas-lms,juoni/canvas-lms,sfu/canvas-lms,sdeleon28/canvas-lms,dgynn/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,skmezanul/canvas-lms,HotChalk/canvas-lms,utkarshx/canvas-lms,efrj/canvas-lms,instructure/canvas-lms,redconfetti/canvas-lms,kyroskoh/canvas-lms,sdeleon28/canvas-lms,matematikk-mooc/canvas-lms,tgroshon/canvas-lms,matematikk-mooc/canvas-lms,tgroshon/canvas-lms,efrj/canvas-lms,instructure/canvas-lms,juoni/canvas-lms,fronteerio/canvas-lms,djbender/canvas-lms,whalejasmine/canvas-lms,grahamb/canvas-lms,skmezanul/canvas-lms,instructure/canvas-lms,whalejasmine/canvas-lms,redconfetti/canvas-lms,fronteerio/canvas-lms,narigone/canvas,matematikk-mooc/canvas-lms,skmezanul/canvas-lms,narigone/canvas,grahamb/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,greyhwndz/canvas-lms,venturehive/canvas-lms,snehakachroo1/LSI_replica,instructure/canvas-lms,juoni/canvas-lms,efrj/canvas-lms,dgynn/canvas-lms,leftplusrightllc/canvas-lms,redconfetti/canvas-lms,djbender/canvas-lms,dgynn/canvas-lms,dgynn/canvas-lms,sdeleon28/canvas-lms,SwinburneOnline/canvas-lms,AndranikMarkosyan/lms,antoniuslin/canvas-lms,tgroshon/canvas-lms,leftplusrightllc/canvas-lms,SwinburneOnline/canvas-lms,tgroshon/canvas-lms,juoni/canvas-lms,SwinburneOnline/canvas-lms,kyroskoh/canvas-lms,SwinburneOnline/canvas-lms,grahamb/canvas-lms,utkarshx/canvas-lms,skmezanul/canvas-lms,instructure/canvas-lms,roxolan/canvas-lms,kyroskoh/canvas-lms,jay3126/canvas-lms,snehakachroo1/LSI_replica,grahamb/canvas-lms,narigone/canvas,kyroskoh/canvas-lms,greyhwndz/canvas-lms,roxolan/canvas-lms