[DISCUSS] Should we change TieredMergePolicy's segment deletion accounting to use numDocs in the denominator rather than MaxDoc?

### Description

[TieredMergePolicy](https://github.com/apache/lucene/blob/branch_9_8/lucene/core/src/java/org/apache/lucene/index/TieredMergePolicy.java#L382) use `MaxDoc` to calculate the `segDelPct`. There will be scenario that developers expect a segment deletion pct to be `delCount / (maxDoc-delCount)` and this accounting seems more realistic than current accounting.

Start at TieredMergePolicy.java around the segDelPct calculation linked in the issue. Read the surrounding merge-policy logic and the issue discussion to understand the current MaxDoc accounting and the proposed numDocs denominator. Done would require a decided direction and corresponding implementation and tests, none of which are specified here.

Written by the indexing model from the issue text.