docs: clean up router api doc (#31476)

PR Close #31476

tools/public_api_guard/router/router.d.ts

@@ -255,6 +255,8 @@ export declare const PRIMARY_OUTLET = "primary";
 
 export declare function provideRoutes(routes: Routes): any;
 
+export declare type QueryParamsHandling = 'merge' | 'preserve' | '';
+
 export interface Resolve<T> {
     resolve(route: ActivatedRouteSnapshot, state: RouterStateSnapshot): Observable<T> | Promise<T> | T;
 }