[MRG+1] ENH/MNT Rename labels --> groups in CV tools (#6660)

doc/modules/cross_validation.rst

doc/tutorial/statistical_inference/model_selection.rst

doc/whats_new.rst

sklearn/model_selection/__init__.py

sklearn/model_selection/_search.py

sklearn/model_selection/_split.py

sklearn/model_selection/_validation.py

sklearn/model_selection/tests/test_search.py

sklearn/model_selection/tests/test_split.py

sklearn/model_selection/tests/test_validation.py