Improve set deletions percentage javadoc (#12828)

Co-authored-by: Yuan,Xiao <yuanxxx@amazon.com>

lucene/core/src/java/org/apache/lucene/index/TieredMergePolicy.java

@@ -157,9 +157,10 @@ public double getMaxMergedSegmentMB() {
   }
 
   /**
-   * Controls the maximum percentage of deleted documents that is tolerated in the index. Lower
-   * values make the index more space efficient at the expense of increased CPU and I/O activity.
-   * Values must be between 5 and 50. Default value is 20.
+   * Sets the maximum percentage of doc id space taken by deleted docs. The denominator includes
+   * both active and deleted documents. Lower values make the index more space efficient at the
+   * expense of increased CPU and I/O activity. Values must be between 5 and 50. Default value is
+   * 20.
    *
    * <p>When the maximum delete percentage is lowered, the indexing thread will call for merges more
    * often, meaning that write amplification factor will be increased. Write amplification factor