DOC: Standardize the module documentation format (work in progress)

doc/modules/classes.rst

sklearn/cluster/__init__.py

sklearn/covariance/__init__.py

sklearn/cross_validation.py

sklearn/datasets/__init__.py

sklearn/decomposition/__init__.py

sklearn/ensemble/__init__.py

sklearn/feature_extraction/__init__.py

sklearn/feature_extraction/image.py

sklearn/feature_extraction/text.py

sklearn/feature_selection/__init__.py

sklearn/gaussian_process/__init__.py

sklearn/grid_search.py

sklearn/hmm.py

sklearn/lda.py

sklearn/linear_model/__init__.py

sklearn/linear_model/sparse/__init__.py