I've seen a large number of patents (and patent applications) in the electronics field. The one quoted in this thread is typical in its detail and style. The patent lawyers I've known are indeed engineers (who for one reason or another went on to law school). It's nerd city, usually, with pocket protectors, and probably a youth spent wearing propeller beanies, but they're amazingly bright. They are quite capable of writing such descriptions after briefly consulting with the inventors. It's the drawings, in my experience, that are done by a specialized technical person.
Probably because of an express desire to become a patent attorney, I imagine - the easiest way to be permitted to sit for the patent bar exam is to have an undergraduate degree in a specified scientific or technical field, as I'm sure you know.