Update documentation references to `init` and `update` functions being returned

[orionarcher]

High-level vs Low-Level
Under the hood, TorchSim takes a modular functional approach to atomistic simulation. Each integrator or optimizer function, such as nvt_langevin, takes in a model and parameters and returns init and update functions that act on a unique State. The state inherits from SimState and tracks the fixed and fluctuating parameters of the simulation, such as the momenta for NVT or the timestep for FIRE. The runner functions take this basic structure and wrap it in a convenient interface with autobatching and reporting.

Given the refactor of the API, this should be updated