chore: add some comments to kafka

src/common/meta/src/wal/kafka.rs

@@ -20,10 +20,14 @@ use serde::{Deserialize, Serialize};
 
 use crate::wal::kafka::topic::Topic;
 
+/// Configurations for bootstraping a kafka wal.
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize, Default)]
 pub struct KafkaConfig;
 
+/// Kafka wal options allocated to a region.
 #[derive(Serialize, Deserialize, Default, Debug, Clone, PartialEq)]
 pub struct KafkaWalOptions {
-    topic: Topic,
+    /// Kafka wal topic.
+    /// Publishers publish log entries to the topic while subscribers pull log entries from the topic.
+    pub topic: Topic,
 }

src/common/meta/src/wal/kafka/topic.rs

@@ -12,4 +12,6 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+/// Kafka wal topic.
+// A topic is simply a string right now. But it may be more complex in the future.
 pub type Topic = String;