chore: Apply suggestions from code review

Co-authored-by: Andrew Lamb <alamb@influxdata.com>

server/src/db/chunk.rs

@@ -201,6 +201,7 @@ impl DbChunk {
         }
     }
 
+    /// Returns the contained `ParquetChunk`, if this chunk is stored as parquet
     pub fn parquet_chunk(&self) -> Option<&Arc<ParquetChunk>> {
         match &self.state {
             State::ParquetFile { chunk } => Some(chunk),