Start migration guide for FormHelper.
Update docs for input().
Move sections around and remove dead sections.

* Move inputs() to the bottom because it is a gross feature.
* Remove sections for methods that do not exist anymore.
Fix title formatting.
Fix invalid meta directive.
Update file docs and update indentation.
Merge pull request #1114 from cakephp/3.0-form-2

3.0 form 2