DOC: Simpler cross-validation iterator doc
2-fold => 2-fold cross-validation
approximately the same percentage of samples
Remove spurious print statements in sample snippets to make the doc easier to follow
Lowercase y and more consistent blank line usage
DOC add missing Returns description
DOC: Update pyamg site and fix a typo.
Merge pull request #2432 from jwkvam/docs_cluster

DOC: Update pyamg site and fix a typo.
Merge pull request #2370 from ogrisel/doc-cross-validation

MRG: DOC: Simpler cross-validation iterator doc
MAINT deprecate indices=False for cross validation generators
DOC boolean masks in CV generators are deprecated
DOC what's new: fast_dot is internal, so don't mention it