Update doc blocks regarding return $this

Refs https://github.com/cakephp/cakephp/pull/3790

also found a few errors like https://github.com/dereuromark/cakephp/compare/cakephp:master...dereuromark:master-docblocks?expand=1#diff-5d7cc9605b8ee08794d01f544518e5d5R1699