Update migration guide for HtmlHelper changes.
Update the HtmlHelper docs for most of the changes in 3.0 thus far.
Update examples, remove dead options, add missing options.
Re-order sections.

Creating buttons and closing the form happen after creating the inputs.
Cover it in the logical order people will need the methods.
Use short array syntax.
Add example of hidden attributes.
Remove data prefix.
Merge pull request #1140 from cakephp/3.0-form-5

3.0 form 5