IIRC, they just served him; there was never a larger background other then that stated fact. There was an official artwork piece, depicting the generals and senshi paired (mentioned earlier in the thread) but nothing was ever directly stated in the original manga, and arguably not even indirectly, but that's another discussion. The reason Venus knows of Kunzite is because of her experiences as Sailor V, in the Sailor V manga. He was the general that Danburite worked under, and this all happened before the events of Sailor Moon, which leads her to do some of the things she does, in the current arc.