Clarify the docstring of cosine_decay_schedule.
Clarify the docstring of cosine_decay_schedule.

The docstring of cosine_decay_schedule did not clearly state that it does not
perform restarts even though the paper it references uses restarts. This commit
clarifies the docstring and explains how to use join_schedules to do restarts.

Fixes issue #304.

PiperOrigin-RevId: 431640180

deepmind/optax,deepmind/optax