clean up ptr docs

r? @steveklabnik @thestinger 
I'm of two minds on the "safety" heading (which I largely removed from these docs other than the intrinsic ones), but I think it's important to call out explicitly Undefined Behaviour, of which this is. That said where Undefined Behaviour starts and stops on the other functions like `read` is a bit complex, which is why I didn't opt for Undefined Behaviour, but I'm not sure on Safety.