docs: clarify flat input schema requirement in compaction reader

Agent-Logs-Url: https://github.com/GreptimeTeam/greptimedb/sessions/d0dc5df7-0d37-4ed0-bfb9-a796fc8025af

Co-authored-by: MichaelScofield <990479+MichaelScofield@users.noreply.github.com>

src/mito2/src/compaction.rs

@@ -1006,6 +1006,8 @@ impl CompactionSstReaderBuilder<'_> {
     /// for compaction. The schema of the [FlatSource] is unified.
     async fn build_flat_sst_reader(self) -> Result<FlatSource> {
         let scan_input = self.build_scan_input().await?.with_compaction(true);
+        // Compaction reads/writes flat SST batches (with internal columns), so this must use
+        // the flat input schema instead of the logical output schema.
         let schema = scan_input.mapper.input_arrow_schema(scan_input.compaction);
 
         let stream = SeqScan::new(scan_input)