add API docs for courses#update

Thanks to eriko for this patch. Adds API docs for courses#update
method, similar to courses#create, but with update-specific
arguments.

Test Plan:
- Check API docs to see that #update method for Courses API has
  arguments.

Fixes SIS-522
Fixes CNVS-8107

Change-Id: Ife403df1a06e6fee522e9a87ec0214210cef9062
Reviewed-on: https://gerrit.instructure.com/40313
Tested-by: Jenkins <jenkins@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>