From the same document
If you are still unable to turn Wi-Fi or Bluetooth on or off after attempting these steps, contact Apple or your wireless carrier for additional assistance.
Could be a hardware problem
I walked into the Apple store today with exactly this problem on iPhone 4S. I had tried everything, and I mean EVERYthing, nothing worked. Genius took one look at phone (greyed out wifi button), said it was irrepairable – logic board failure – and phone had to be replaced. Cost £139 for new handset because out of warranty.