[doc] Various docstring improvements

doc/config.rst

doc/data_model.rst

doc/usage/evaluation.rst

doc/usage/saliency.rst

doc/usage/training.rst

src/mednet/config/data/montgomery/datamodule.py

src/mednet/config/data/montgomery/default.py

src/mednet/config/data/montgomery/fold_0.py

src/mednet/config/data/montgomery/fold_1.py

src/mednet/config/data/montgomery/fold_2.py

src/mednet/config/data/montgomery/fold_3.py

src/mednet/config/data/montgomery/fold_4.py

src/mednet/config/data/montgomery/fold_5.py

src/mednet/config/data/montgomery/fold_6.py

src/mednet/config/data/montgomery/fold_7.py

src/mednet/config/data/montgomery/fold_8.py

src/mednet/config/data/montgomery/fold_9.py

src/mednet/config/data/montgomery_shenzhen/datamodule.py

src/mednet/config/data/montgomery_shenzhen/default.py

src/mednet/config/data/montgomery_shenzhen/fold_0.py