Fix formatting errors.
Start updating the docs for registries.

Collection objects no longer need to be explicitly documented as they
won't really exist in 3.0
Update event docs.

Remove sections that no longer apply and update sections that still do
apply.
Fix wording.
Fix busted links.
Add some migration notes for new collections.
Add example on how to enable/disable callbacks.
Update docs for changes in events and registries.
Update array syntax and App::uses()
Fix array syntax and pathnames.
Update migration guide for TaskCollection.
Update docs and make helper/component docs more similar.
Merge pull request #669 from cakephp/3.0-collection-changes

3.0 collection changes