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
Merge branch 'master' into hc_linkage

Conflicts:
	sklearn/cluster/tests/test_hierarchical.py
BUG: different distance names in scipy
Return scores of all folds

It is possible to plot confidence intervals with fill_between
BUG: fix minor floating-point precision detail