Not sure why this feature isn't turned on by default. It would make all the sense in the world. Also not sure why it's hidden and worded so strangely. In any case, I just did this and recommend it. Even if you don't use Siri, it's useful to keep her silent if you don't want her talking to you when you accidentally enable her in a quiet room.
How to Silence Siri with the Mute Switch on iPhone & iPad | OS X Daily