code: @Override
      public DescribeEventCategoriesResult describeEventCategories(DescribeEventCategoriesRequest request) {
          request = beforeClientExecution(request);
          return executeDescribeEventCategories(request);
      }
docs: <p>
  Displays a list of categories for all event source types, or, if specified, for a specified source type.
  </p>
  
  @param describeEventCategoriesRequest
  @return Result of the DescribeEventCategories operation returned by the service.
  @sample AmazonNeptune.DescribeEventCategories
  @see <a href="http://docs.aws.amazon.com/goto/WebAPI/neptune-2014-10-31/DescribeEventCategories"
  target="_top">AWS API Documentation</a>