📝 Add docstrings to master

Docstrings generation was requested by @till-m.

* #566 (comment)

The following files were modified:

* `bayes_opt/acquisition.py`
* `bayes_opt/bayesian_optimization.py`
* `bayes_opt/target_space.py`
* `tests/test_acquisition.py`
* `tests/test_constraint.py`
* `tests/test_target_space.py`

Author: coderabbitai[bot]