[docs][mongodb] Add docs for MongoDB incremental source

docs/content/connectors/mongodb-cdc.md

@@ -252,14 +252,14 @@ Connector Options
       <td>optional</td>
       <td style="word-wrap: break-word;">false</td>
       <td>Boolean</td>
-      <td>Whether enable parallelism snapshot.</td>
+      <td>Whether enable incremental snapshot. The incremental snapshot feature only supports after MongoDB 4.0.</td>
     </tr>
     <tr>
       <td>scan.incremental.snapshot.chunk.size.mb</td>
       <td>optional</td>
       <td style="word-wrap: break-word;">64</td>
       <td>Integer</td>
-      <td>The chunk size mb of parallelism snapshot.</td>
+      <td>The chunk size mb of incremental snapshot.</td>
     </tr>
     </tbody>
 </table>
@@ -398,8 +398,42 @@ public class MongoDBSourceExample {
 }
 ```
 
-**Note:** If database regex is used, `readAnyDatabase` role is required.
+The MongoDB CDC incremental connector (after 2.3.0) can be used as the following shows:
+```java
+import org.apache.flink.api.common.eventtime.WatermarkStrategy;
+import org.apache.flink.streaming.api.environment.StreamExecutionEnvironment;
+import com.ververica.cdc.connectors.mongodb.source.MongoDBSource;
+import com.ververica.cdc.debezium.JsonDebeziumDeserializationSchema;
+
+public class MongoDBIncrementalSourceExample {
+    public static void main(String[] args) throws Exception {
+        MongoDBSource<String> mongoSource =
+                MongoDBSource.<String>builder()
+                        .hosts("localhost:27017")
+                        .databaseList("inventory") // set captured database, support regex
+                        .collectionList("inventory.products", "inventory.orders") //set captured collections, support regex
+                        .username("flink")
+                        .password("flinkpw")
+                        .deserializer(new JsonDebeziumDeserializationSchema())
+                        .build();
+
+        StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();
+        // enable checkpoint
+        env.enableCheckpointing(3000);
+        // set the source parallelism to 2
+        env.fromSource(mongoSource, WatermarkStrategy.noWatermarks(), "MongoDBIncrementalSource")
+                .setParallelism(2)
+                .print()
+                .setParallelism(1);
+
+        env.execute("Print MongoDB Snapshot + Change Stream");
+    }
+}
+```
 
+**Note:** 
+- If database regex is used, `readAnyDatabase` role is required.
+- The incremental snapshot feature only supports after MongoDB 4.0.
 
 Data Type Mapping
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