Add doc updates for fallbacks

I also cleaned up the SEO code example, which arguably promoted a bad practice (`*/*/*` route pattern).

Allows merging:

https://github.com/cakephp/cakephp/pull/5199
https://github.com/cakephp/app/pull/176

Closes:

https://github.com/cakephp/cakephp/issues/5340