This doesn't seem like a totally unreasonable request. Especially if you are applying from a smaller school, or getting a recommendation from a more junior faculty member, the person writing the letter might not have *that* much experience. I was asked to write two letter s f recommendation for students in my calculus class while doing my phd, and let me tell you I had no idea what I was doing. I also asked a postdoc who taught a 500 level graph theory course for a letter of recommendation to a summer program and he was caught pretty off guard - I think it was his first time doing it.
Ideally if they don't know what to include they would ask a more senior faculty member, but if you happen to know what the contents should look like I don't know why saying something like "I'm applying to xyz, and I would really appreciate it if you could write a letter of recommendation talking about my ability to do abc" would offend anyone. They might think abc is the wrong thing to focus on.