[skip ci] Fix root key documentation (rails-api#1761)

The current documentation stated that it was not possible to override
the root key for the JSON adapter. This was removed from the
documentation and it now links the useful documentation pages.

docs/general/adapters.md

@@ -67,9 +67,11 @@ Doesn't follow any specific convention.
 
 ### JSON
 
-The response document always with a root key.
+The json response is always rendered with a root key.
 
-The root key **can't be overridden**, and will be derived from the resource being serialized.
+The root key can be overridden by:
+* passing the `root` option in the render call. See details in the [Rendering Guides](rendering.md#overriding-the-root-key).
+* setting the `type` of the serializer. See details in the [Serializers Guide](serializers.md#type).
 
 Doesn't follow any specific convention.