*: document auto-compaction logic

[gyuho]

Related to #9443.

This summarizes how our auto-compaction logic has been changed.

/cc @fanminshi @yudai

[gyuho]

Current auto-compaction logic makes sense to me, but it gets too aggressive when the compaction period is less than 1-hour (as discussed in #9443). We could add additional minimum value checks.

[yudai]

For 3.2, I think the description is correct :)

[codecov-io]

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