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.