Motivation

This is a follow-up PR for #6119.
I think this is optional since the _get_best_trial docstring is not rendered in a reference.

Description of the changes

• Updated the docstring to follow the existing code base, such as

  optuna/optuna/study/study.py

  Lines 255 to 264 in 8f053dc

  	deepcopy:
  	Flag to control whether to apply ``copy.deepcopy()`` to the trials.
  	Note that if you set the flag to :obj:`False`, you shouldn't mutate
  	any fields of the returned trial. Otherwise the internal state of
  	the study may corrupt and unexpected behavior may happen.
  	states:
  	Trial states to filter on. If :obj:`None`, include all states.
  	Returns:
  	A list of :class:`~optuna.trial.FrozenTrial` objects.