[DOC] Add docstrings for best_estimator.py

[omkar-334]

Description

This PR adds docstrings for best_estimator.py, from https://github.com/scikit-learn/scikit-learn/blob/main/sklearn/model_selection/_search.py

Related Issues

Fixes #226

Type of Change

• [BUG] - Bug fix (non-breaking change fixing an issue)
• [ENH] - New feature (non-breaking change adding functionality)
• [DOC] - Documentation changes
• [MNT] - Maintenance

How was this solved?

Checklist

• PR title includes appropriate tag: [BUG], [ENH], [DOC] or [MNT]
• Linked to related issue (if applicable)
• Code passes make check (lint, format, isort)

Note

@SimonBlanke make check is failing because of unused imports, unorganized imports and other issues... Can I take that up too?

Found 90 errors.
[*] 67 fixable with the `--fix` option (1 hidden fix can be enabled with the `--unsafe-fixes` option).
make: *** [lint] Error 1