Mention that Orbot is no longer required to run Onion Browser on iOS

With version 3.0 of Onion Browser, users also need to install Orbot. We should mention this in our documentation: "Can I run Tor Browser on an iOS device?"

Update: It's no longer required to have Orbot + Onion Browser, but if you have Orbot installed, Onion Browser will ask permission to use it. We will need to update the documentation.