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. https://github.com/rust-lang/rust/issues/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
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