The pixhawk itself can't detect if the receiver has lost signal that would be a feature of the receiver itself. It can however detect if it's lost contact with then ground station if you have one and be set to RTL. If you want a TX failsafe you'll need to buy a receiver that has a TX signal loss failsafe and set that up according to it's instructions. Hope that helps.
Here's how failsafe works on mine. I bought a fr sky receiver that has a fail safe option. To set this up you set your transmitter to the exact way you want it for fail safe. So for instance I have RTL set up on channel 7. So I turn that on on my transmitter, then press a little 'set' button on the receiver. So now if it flies out of range the receiver will set RTL for me when it detects loss of TX signal. But notice none of that has anything to do with the APM or Pixhawk. It doesn't know the difference between me telling channel 7 to set RTL or the receiver setting RTL on channel 7. So as you see it's really not the APM that has a fail safe it's the receiver for you transmitter.
Many receivers didn't have the failsafe option or a very limited one called throttle fail safe. As was the case with my Futaba 7C, it only has throttle failsafe. That can be set up with the APM too but it's a bit more complicated to explain. That link you posted above covers it. But for only about $35 the receiver I got is much better than simple throttle failsafe and as easy to set up as pressing a button. Another thing that isn't so good about throttle fail safe is that won't work with a situation I have with a gas plane. In order for the APM to detect fail safe the throttle channel has to go below idle speed. Well with a gas engine that shuts it off!! So it's pointless. With an electric craft that might not be a problem. But again... for $35 screw throttle failsafe.
The fail safe receiver I got was a FR Sky TFR4 SB which works with my Futaba 7C FASST system. http://www.hobbyking.com/hobbyking/store/%5F%5F27176%5F%5FFrSky%5FT...
What radio do you have? I'm sure you can find a receiver that has failsafe that will work.
This is an old post but I would also like to be educated further.
How much throttle is require for RTL failsafe?
Thanks in advance