Improve `Step` docs
It [came up on urlo](https://users.rust-lang.org/t/implement-trait-step-in-1-76-0/108204?u=cad97) that the unstable reason string isn't helpful, so just remove it; there's nothing meaningful to add here.
Also makes a couple drive-by improvements to the method docs -- removes incorrect references, changes `forward_checked`'s invariant formulation to match `backward_checked`'s, and adds a helpful corollary that `step_unchecked(a, 0)` is always safe.
a0e50a4a96