Improve how users find the context of options

From #4936, given the name of an option, e.g. number-sections, it can be hard for a user to understand the context it needs to be used in, e.g. under a format not under book.

It could be:

• This is an educational need, i.e. some explanation somewhere that teaches people how to figure out the right context.
• This is a "Reference" doc improvement, e.g. options have a more explicit link to their context.
  • Is the page they occur on (e.g. number-sections is on formats/html.html) sufficient?
• This is a "Guide" doc improvement, e.g. when options are presented context needs to be made more explicit (or there needs to be a way for a user to easily find the context directly from the Guide page).