To make my comment bank, I copied comments as I graded a set of papers onto a doc. After several grading sessions, I had dozens of comments pasted. I then organized them into separate categories (mechanics, usage, grammar, style, organization, research issues) to make them easy to find. I also added links to websites and videos to allow students to get tutorials on each writing issue. So instead of students just getting "comma splice" as a comment, they also get a link to the Purdue Online Writing Lab website where the issue is explained in detail.
Many tips and tricks make the Google Doc essay grading process work, but developing a comment bank document is invaluable. Here is my comment bank. Feel free to use and adapt it.