This is an absolute must have feature for the reasons so many others have pointed out. I don't feel good about putting stuff out there without making everything that is happening clear to anyone (including myself) that works with the solution later. Also a separate code window, perhaps something similar to the VBA editor in Excel is sorely needed.
See the blog post here: https://powerapps.microsoft.com/en-us/blog/comment-your-powerapps-code/ for details on how to use this.