Explain how the LSS file storage works in relation to compaction

There are warnings in both the docs and Web UI that auto compaction does not apply to GSI indexes, but there's no explanation as to how compaction actually works for indexes. Customers have raised a few issues about index disk size seemingly increasing for no reason as well as high fragmentation. This could be mitigated by an explanation of how the standard index storage method works, where the compaction thresholds are, and the ways the change them.