Clarifying VersionedIntervalTimeline.findFullyOvershadowed()

In #7547, @jihoonson added clarifying comments for `completePartitionsTimeline` and `incompletePartitionsTimeline` fields in `VersionedIntervalTimeline`. One of the comments say that "IncompletePartitionsTimeline also includes completePartitionsTimeline".

It explains to me why the second part of the `findFullyOvershadowed()` algorithm iterates only over `incompletePartitionsTimeline`:

https://github.com/apache/incubator-druid/blob/7b9bd5359db80bdd9bb3f547161f94b369469b76/core/src/main/java/org/apache/druid/timeline/VersionedIntervalTimeline.java#L392-L404

But it doesn't explain why then the first part does exactly the same loop over both `completePartitionsTimeline` and `incompletePartitionsTimeline`:

https://github.com/apache/incubator-druid/blob/7b9bd5359db80bdd9bb3f547161f94b369469b76/core/src/main/java/org/apache/druid/timeline/VersionedIntervalTimeline.java#L364-L382

Related to #7433 and #8070.

Start in core/src/main/java/org/apache/druid/timeline/VersionedIntervalTimeline.java and read findFullyOvershadowed(), especially the loops around lines 364-404 and the comments for completePartitionsTimeline and incompletePartitionsTimeline. Clarify why the first loop iterates over both timelines while the second iterates only over incompletePartitionsTimeline; the comments should make this distinction understandable.

Written by the indexing model from the issue text.