Document controller APIs

[pablobm]

When reviewing #1478, it became apparent to me that controller APIs such as resource_params are not documented.

I think they should be part of Administrate's interface, and as such should be documented. Here's some documentation.

Additionally:

• Changed the example for update so that it's more useful. The current example doesn't render or redirect, so it may cause confusion to someone trying to emulate it.
• Mentioned requested_resource too.
• Fixed the indent for the scoped_resource example.

[sedubois]

NB: still needs to be deployed to https://administrate-prototype.herokuapp.com/customizing_controller_actions.