I always leave explanations for the editing changes I have made. I do this by copying and pasting the original sentence (shown as Before) and then paste it again to show the grammar corrections (shown as After). Above that, I briefly mention what change(s) was (were) made and then link to a web page on a reputable grammar site that explains the specific grammar rule.
I don't think I am doing the work of a mentor because any explanations I make revolve solely around grammar. I do this to help the contributor in case s/he is interested in improving his or her grammar. After all, once the contributor reaches a certain rank, s/he will no longer have someone to look over his or her review before publishing. The linked grammar sites I provide would be an invaluable resource to anyone willing to spend a few moments looking over them. Granted, not everyone is interested in improving his or her grammar, but I have received enough appreciative feedback to know that at least many of the contributors welcome the information.
If the contributor tends to make a certain grammar error repetitively, wouldn't it be beneficial to him or her to know about it? If the contributor is not aware of making a specific error, s/he may not notice that it has been corrected. And on the flip side, if I correct it without a brief explanation and a link to a site explaining the grammar rule (a grammar rule of which the contributor may be unaware), then the contributor will not have learned anything or may think that a correction was gratuitous. Additionally, once the contributors being edited reach a certain rank, the site will just have that many more reviews with grammar errors if they are not helped along the way during the time period when they are receiving editing services.
That's how I view what I do: a service not only to benefit the readers but a service to benefit contributors as well. I want each review that I edit to be as grammatically correct as I am capable of making it, and I want to provide the tools to help the contributor grammatically correct and polish his or her own reviews as well. As the saying goes: If you want to feed a man for one day, give him a fish. If you want to feed him for a lifetime, teach him to fish . . . or something like that!
I just want to provide the resources contributors could use to improve the grammar in their future reviews (not to mention how much these resources might help them in all the other areas of their lives in which written communication is required).