When you are in photos.google.com and you select a photo you can click on the white circle with the I. And then type comments where it says Type a description.
The problem that I have found is that the system works differently when you are looking at the photos in google drive, not google photos. Google drive also has a way to enter info into the photos, but that info is not shared between google drive's photos and google photos, which are the same photos.
I wish there was a way to enter the info into the .jpg.