[Dubbo-5200] fix javadoc for Serialization.getContentTypeId (#5201)

dubbo-serialization/dubbo-serialization-api/src/main/java/org/apache/dubbo/common/serialize/Serialization.java

@@ -36,7 +36,9 @@ import java.io.OutputStream;
 public interface Serialization {
 
     /**
-     * Get content type unique id, recommended that custom implementations use values greater than 20.
+     * Get content type unique id, recommended that custom implementations use values different with
+     * any value of {@link Constants} and don't greater than ExchangeCodec.SERIALIZATION_MASK (31) 
+     * because dubbo protocol use 5 bits to record serialization ID in header.
      *
      * @return content type id
      */
@@ -71,4 +73,4 @@ public interface Serialization {
     @Adaptive
     ObjectInput deserialize(URL url, InputStream input) throws IOException;
 
-}
+}