Improve interface and documentation
Mocks inherit from BaseEstimator
Correct docstring
Add narrative documentation
Improve documentation

* add images
* add link to API doc
* change order
Fix link to validation curve
Add example with polynomial regression
Correct and improve documentation
Improve phrasing
Rephrase sentence
Simplify first part of the documentation
Merge pull request #2768 from robertlayton/meanshiftdoc

MRG: Update to Mean Shift clustering documentation
COSMIT remove unused imports and variables
Minor doc example fix
Merge pull request #2820 from ankit-maverick/issue2819

Minor DecisionTree doc example fix
Merge pull request #2818 from jnothman/flakes

COSMIT remove unused imports and variables
Fixed FS1995 citation.
Merge pull request #2825 from bryan-lunt/master

Fixed FS1995 citation.
MISC plot_rfe_with_cross_validation: better comments

Fixes #2817
Fix typo