define fetch policies as union

src/core/watchQueryOptions.ts

@@ -16,6 +16,16 @@ import {
   PureQueryOptions,
 } from './types';
 
+  /**
+   * fetchPolicy determines where the client may return a result from. The options are:
+   * - cache-and-network: returns result from cache first (if it exists), then return network result once it's available
+   * - cache-first: return result from cache. Only fetch from network if cached result is not available.
+   * - network-first: return result from network, but if network request fails, use result from cache, if available.
+   * - cache-only: return result from cache if avaiable, fail otherwise.
+   * - network-only: return result from network, fail if network call doesn't succeed.
+   */
+export type FetchPolicy = 'cache-first' | 'cache-and-network' | 'network-first' | 'cache-only' | 'network-only';
+
 /**
  * We can change these options to an ObservableQuery
  */
@@ -46,14 +56,9 @@ export interface ModifiableWatchQueryOptions {
   pollInterval?: number;
 
   /**
-   * fetchPolicy determines where the client may return a result from. The options are:
-   * - cache-and-network: returns result from cache first (if it exists), then return network result once it's available
-   * - cache-first: return result from cache. Only fetch from network if cached result is not available.
-   * - network-first: return result from network, but if network request fails, use result from cache, if available.
-   * - cache-only: return result from cache if avaiable, fail otherwise.
-   * - network-only: return result from network, fail if network call doesn't succeed.
+   * Specifies the {@link FetchPolicy} to be used for this query
    */
-  // fetchPolicy?: string;
+  fetchPolicy?: FetchPolicy;
 
   /**
    * Whether or not updates to the network status should trigger next on the observer of this query