Rollup merge of rust-lang#22030 - mdinger:f32_examples, r=steveklabnik

 Some examples for `std::num::Float`

~~This is WIP for making examples for `f32`. This probably won't pass `make tidy` and I'm not sure which `f32` needs documentation. rust-lang#22025 shows 2 sets of `f32` which seems split between `core` and `std`. I'm not sure which should be documented but I started doing a couple from `std`. Easy to move if that's where they go...~~

~~Gotta build it eventually to actually see if the docs actually appear where I think they will or if I'm just disillusioned.~~

cc @steveklabnik

Author: Manishearth