add javadoc to Jackson.java (#823)

* add javadoc to Jackson.java

* Update Jackson.java

Author: pjfanning

http-marshallers-java/http-jackson/src/main/java/org/apache/pekko/http/javadsl/marshallers/jackson/Jackson.java

@@ -37,6 +37,7 @@
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.fasterxml.jackson.databind.json.JsonMapper;
 
+/** A JSON marshaller/unmarshaller using the Jackson library. */
 public class Jackson {
   private static final ObjectMapper defaultObjectMapper =
       createMapper().enable(MapperFeature.SORT_PROPERTIES_ALPHABETICALLY);