Add docs for new decorators

Author: qqii

docs/api-guide/views.md

@@ -186,6 +186,9 @@ The available decorators are:
 * `@authentication_classes(...)`
 * `@throttle_classes(...)`
 * `@permission_classes(...)`
+* `@content_negotiation_class(...)`
+* `@metadata_class(...)`
+* `@versioning_class(...)`
 
 Each of these decorators takes a single argument which must be a list or tuple of classes.