docs: clarify that caller renames merge output to match generated split ID

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: g-talbot

quickwit/quickwit-parquet-engine/src/merge/metadata_aggregation.rs

@@ -104,6 +104,9 @@ pub fn merge_parquet_split_metadata(
         .expect("at least one input")
         + 1;
 
+    // The generated split ID determines the expected filename. The caller
+    // (ParquetMergeExecutor) renames the merge engine's output file to match
+    // this name before handing it to the uploader.
     let split_id = ParquetSplitId::generate(first.kind);
     let parquet_file = format!("{split_id}.parquet");