[TSD] [SE-3988] add annotations to instructor plugin settings
This PR adds standardized settings documentation for the instructor plugin settings.
*JIRA tickets*: TBD
*Merge deadline*: "None"
*Author notes and concerns*:
I'm not sure I've used the opt-in/opt-out annotation correctly. If not, please let me know and I'll fix it.
[ ] TBD
[ ] edX reviewer[s] TBD