fix comment after talked to Boaz

server/src/main/java/org/elasticsearch/index/engine/InternalEngine.java

@@ -1390,18 +1390,19 @@ public boolean shouldPeriodicallyFlush() {
             return false;
         }
         /*
-         * We flush to reduce the size of uncommitted translog but strictly speaking the uncommitted size won't always be below
-         * the threshold after a flush. An endless loop may occur when the uncommitted size is close to the flush threshold,
-         * the current generation is also close to the generation threshold, and an index is faster and rolls a new generation.
+         * We flush to reduce the size of uncommitted translog but strictly speaking the uncommitted size won't always be
+         * below the flush-threshold after a flush. To avoid getting into an endless loop of flushing, we only enable the
+         * periodically flush condition if this condition is disabled after a flush. The condition will change if the new
+         * commit points to the later generation the last commit's(eg. gen-of-last-commit < gen-of-new-commit)[1].
          *
-         * Avoid getting into an endless loop of flushing, we only enable the periodically flush condition if this condition is disabled
-         * after a flush. The condition will change if the new commit points to the later generation the last commit's.
-         * Or if the local checkpoint equals to max_seqno and translogGenerationOfLastCommit equals to translogGenerationOfNewCommit,
-         * we know that the last generation must contain operations because its size is above the flush threshold and
-         * the flush threshold is guaranteed to be higher than an empty translog by the setting validation.
-         * This guarantees that the new commit will point to the newly rolled generation.
+         * When the local checkpoint equals to max_seqno, and translog-gen of the last commit equals to translog-gen of
+         * the new commit, we know that the last generation must contain operations because its size is above the flush
+         * threshold and the flush-threshold is guaranteed to be higher than an empty translog by the setting validation.
+         * This guarantees that the new commit will point to the newly rolled generation. In fact, this scenario only
+         * happens when the generation-threshold is close to or above the flush-threshold; otherwise we have rolled
+         * generations as the generation-threshold was reached, then the first condition (eg. [1]) is already satisfied.
          *
-         * This method is to maintain translog only, thus the IndexWriter#hasUncommittedChanges condition is not considered.
+         * This method is to maintain translog only, thus IndexWriter#hasUncommittedChanges condition is not considered.
          */
         final long translogGenerationOfNewCommit =
             translog.getMinGenerationForSeqNo(localCheckpointTracker.getCheckpoint() + 1).translogFileGeneration;