Update validation docs to reflect recent changes.

Validation methods changed in cakephp/cakephp#3435, sync up the docs.
Propagate validation changes to other parts of the docs.
Adding some notes about changes in the Time lib
Some migration notes for TreeBehavior
Method annotations in Collection docs
Adding docs for Collection::nest and Collection::listNested
Merge pull request #1324 from cakephp/3.0-migration-notes

3.0 migration notes
Fixed typo
Migration notes for API changes
spelling
doc improvements around auth
cs
Merge pull request #1326 from dereuromark/3.0-api

3.0 api
Merge pull request #1325 from cakephp/3.0-collection-additions

3.0 collection additions
Merge pull request #1327 from cakephp/3.0-validation-update

3.0 validation update