Improve the documentation of 'declearn.fairness'.

declearn/fairness/__init__.py

declearn/fairness/api/_client.py

declearn/fairness/api/_dataset.py

declearn/fairness/api/_server.py

declearn/fairness/core/_inmemory.py

declearn/fairness/fairbatch/__init__.py

declearn/fairness/fairbatch/_server.py

declearn/fairness/fairfed/__init__.py

declearn/fairness/fairfed/_server.py

declearn/fairness/fairgrad/__init__.py

declearn/fairness/fairgrad/_server.py

declearn/fairness/monitor/_server.py