RESTful Routing needs to mention the ability to use a `prefix` for a Resource

[mehov]

The documentaton at routing.html#restful-routing (github source file) needs to mention the prefix option available for a resource introduced by cakephp/cakephp/pull/8493 in a response to cakephp/cakephp#8063

[markstory]

Docs were updated in 3d6515c (3.x) and 6c136b3 (4.x)