Explain a difference between interval and trueInterval in TimelineObjectHolder

https://github.com/apache/incubator-druid/pull/7306#discussion_r272737017 here @egor-ryashin raised a valid concern. I cannot make sense of it either.

FYI @jihoonson

Start with TimelineObjectHolder and the linked discussion in pull request #7306, especially the concern raised there about interval and trueInterval. Review the surrounding usage to determine the distinction, then document that explanation where the project keeps relevant documentation or comments. Done means the difference is clearly stated and addresses the discussion's concern.

Written by the indexing model from the issue text.