Revise documentation now that quests can have different numbers of threads

[wjt]

With #1234 we will certainly have to update some documentation:

• Search the discussion board, find and update stuff there
• Sync with learning team to update learning materials

[wjt]

https://github.com/endlessm/threadbare/wiki/How-to-make-a-StoryQuest-where-you-only-collect-1-or-2-threads updated

[wjt]

I checked https://studio.endlessstudios.com/programs/1737709/?progTab=Topics and could not find any references to the documentation above, or more generally to how to change the total number of threads in a quest.

@endlessm/learning Are there any learning materials that need to be updated now that there is a better way to change the total number of threads in a quest?