How about: "The GPL (V2 or V3) requires anyone who distributes your code - or a derivative work - to make the source available. V3 is similar to V2, but it also demands that, whenever the code comes "pre-installed" in a device, the user can run a modified version in the device"
Or even: "The GPL (V2 or V3) requires anyone who distributes your code - or a derivative work - to make the source available. V3 is similar to V2. Only worry about the difference for code that comes "pre-installed" in a device"