Add prototype for validation curve
Add documentation and tests
Improve test coverage

... to 99%
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
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.