Fix documentation to explain how schedules work in optimizers.

[rosshemsley]

Discussed in #390

^{Originally posted by nalzok August 13, 2022}
For example, in the documentation for optax.adam, we have

learning_rate (Union[float, Callable[[Union[ndarray, float, int]], Union[ndarray, float, int]]]) – this is a fixed global scaling factor.

I noticed that learning_rate can be a function. How can a function be "a fixed global scaling factor"? What parameters should such a function take, and how does its return value affect the optimization process given that Adam has a learning rate schedule on its own?

[vroulet]

Done in #778