Add docs for track_events with multiple values

The following discussion from !46853 (merged) should be addressed:

• @reprazent started a discussion: (+2 comments)

  Is the idea that these feature flags remain in our codebase forever?

  I do think it makes sense to have a link between an event and a feature flag here. But I think once we've seen that everything is fine, we should be able to remove the feature flag.

  Should we document that?