Document why we need `lastPosBlockOffset` (#12541)

* Document why we need `lastPosBlockOffset`

* Let ./gradlew tidy fix the formatting

* Fix '<' with &lt;

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Co-authored-by: Tony Xu <tonyx@amazon.com>

lucene/core/src/java/org/apache/lucene/codecs/lucene90/Lucene90PostingsFormat.java

@@ -455,6 +455,12 @@ public final class Lucene90PostingsFormat extends PostingsFormat {
     /**
      * file offset for the last position in the last block, if there are more than {@link
      * ForUtil#BLOCK_SIZE} positions; otherwise -1
+     *
+     * <p>One might think to use total term frequency to track how many positions are left to read
+     * as we decode the blocks, and decode the last block differently when num_left_positions &lt;
+     * BLOCK_SIZE. Unfortunately this won't work since the tracking will be messed up when we skip
+     * blocks as the skipper will only tell us new position offset (start of block) and number of
+     * positions to skip for that block, without telling us how many positions it has skipped.
      */
     public long lastPosBlockOffset;
     /**