Add docs for preferFetchable argument

Reviewed By: antoinebrault

Differential Revision: D58152287

fbshipit-source-id: 142e6685698ca6632744c3151d99851ad11338b1

website/docs/api-reference/graphql/graphql-directives.md

@@ -110,12 +110,16 @@ Notes:
 
 With `usePaginationFragment`, Relay expects connection fields to be annotated with a `@connection` directive. For more detailed information and an example, check out the [docs on `usePaginationFragment`](../../guided-tour/list-data/rendering-connections).
 
-## `@refetchable(queryName: String!, directives: [String])`
+## `@refetchable(queryName: String!, directives: [String], preferFetchable: Boolean)`
 
 With `useRefetchableFragment` and `usePaginationFragment`, Relay expects a `@refetchable` directive. The `@refetchable` directive can only be added to fragments that are "refetchable", that is, on fragments that are declared on `Viewer` or `Query` types, or on a type that implements `Node` (i.e. a type that has an id). The `@refetchable` directive will autogenerate a query with the specified `queryName`. This will also generate Flow types for the query, available to import from the generated file: `<queryName>.graphql.js`. For more detailed information and examples, check out the docs on [`useRefetchableFragment`](../use-refetchable-fragment/) or [`usePaginationFragment`](../use-pagination-fragment/).
 
 Optionally, you can pass in a list of directives to add to the autogenerated query. For example, this can be used to add the `@relay_test_operation` directive for [testing](../../guides/testing-relay-components):
 
+[Optional] `preferFetchable: Boolean`
+
+This argument tells the Relay compiler to prefer generating `fetch_MyType(): MyType` queries for types that implement the `Node` interface. This is useful for schemas that have adopted the `@strong` and `@fetchable` server annotations for types. You can directly fetch concrete objects without needing to refine `Node` interface to a specific type.
+
 ```javascript
 graphql`
   fragment FriendsListComponent_user on User