code: public JsonDocument append(String documentId, String json) {
          this.bucket.append(JsonDocument.create(documentId, JsonObject.fromJson(json)));
          return this.get(documentId);
      }
docs: Appends a {@link JsonDocument}'s content to an existing one.
  
  @param documentId the unique ID of the document
  @param json       the JSON String representing the document to append
  @return the updated {@link Document}
  @see Bucket#append(Document)