message: docs(map): add comments
diff: @@ -49,8 +49,12 @@ export function map<T, R>(project: (value: T, index: number) => R, thisArg?: any
  const subscriber = this;
  // The index of the value from the source. Used with projection.
  let index = 0;
  + // Subscribe to the source, all errors and completions are sent along
  + // to the consumer.
  source.subscribe(
  new OperatorSubscriber(subscriber, (value: T) => {
  + // Call the projection function with the appropriate this context,
  + // and send the resulting value to the consumer.
  subscriber.next(project.call(thisArg, value, index++));
  })
  );
  