Add docstrings to optuna.termintor

[Alnusjaponica]

Motivation

Resolve #4598

Description of the changes

• Add docstrings to
  • TerminatorCallback (optuna/terminator/callback.py)
  • CrossValidationErrorEvaluator (optuna/terminator/erroreval.py)
  • report_cross_validation_scores (optuna/terminator/erroreval.py)
  • StaticErrorEvaluator (optuna/terminator/erroreval.py)
• Fix docstring of Terminator (optuna/terminator/terminator.py)
• Add .rst files for terminator documentation

[Alnusjaponica]

There are two points to discuss regarding this PR:

• To avoid errors in doctest, the output destination of the print statement in the Terminator docstring is set to standard error. However, I don't think this is a good approach. What should be done instead?
• Is the current module hierarchy and the correspondence with the .rst files correct?
  I am going to update this PR or submit another follow-up PR according to how Add import for public API of optuna.terminator to optuna/terminator/__init__.py #4669 progresses.

[codecov-commenter]

Codecov Report

Merging #4674 (829e4b3) into master (466cddf) will decrease coverage by 0.01%.
The diff coverage is n/a.

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@@            Coverage Diff             @@
##           master    #4674      +/-   ##
==========================================
- Coverage   90.62%   90.61%   -0.01%     
==========================================
  Files         187      186       -1     
  Lines       14297    14292       -5     
==========================================
- Hits        12956    12951       -5     
  Misses       1341     1341              

Impacted Files	Coverage Δ
optuna/terminator/callback.py	100.00% <ø> (ø)
optuna/terminator/erroreval.py	97.67% <ø> (ø)
optuna/terminator/terminator.py	96.66% <ø> (ø)

... and 3 files with indirect coverage changes

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