Collaborative features are a big part of the draw to Google's online office suite. Part of this is the ability for multiple people to edit documents from two locations at once. Since this can get hectic really fast, another important part is being able to communicate using comments. Now mobile comments look the same across Android and iOS in Google Docs, Sheets, and Slides. Whichever app you're using, you can communicate in real-time.
Google Apps customers are often left in the cold, but this functionality is available to them as well. Users can also add a teammate to a conversation by typing their name in a comment. And on desktops, users can insert comments by clicking on the bubble on the right side of the screen.
For more details, hit up the source links.
You can also find more specifics in this Google Apps Updates post. The new features should be available to everyone within 1 to 3 days.