New beauty pages(404, 422, 500)

moving changelog note to unreleased section

New beauty pages(404, 422, 500)
Merge pull request #9512 from Jacke/master

New 404, 422, 500 pages, they are more stylish and bring ruby essence…
integrate the strong params README into the AC guide.

The current ActionController guide does not mention strong parameters
at all. I integrated the README into the guide to explain the API.

I also included a section to illustrate that the API does not solve
all possible whitelisting scenarios.

The origin was #9454.
Merge pull request #9465 from senny/document_strong_params_in_ac_guide

integrate the strong params README into the AC guide.