One of the easiest ways to give feedback in a Google Doc is to use the comments tool in the doc itself.  You can go a step further by using Kaizena to record audio while you are highlighting text or by inserting comments and resources to help the student improve.  Sometimes we can provide better feedback by speaking to the student than just by writing it out.  Kaizena integrates with your Drive so you can easily access student work and the feedback is saved so you can always come back to it later, something that may not happen if you just write the comments on their assignment and hand it back, never to be seen again. (Link to info about the new features to Kaizena)