[docs] Update Rhai docs around response headers (#3927)

This is called out in our docs so it should probably live in the
examples too

docs/source/customizations/rhai-api.mdx

@@ -528,7 +528,9 @@ The following fields are identical in behavior to their `request` counterparts:
 * [`body`](#requestbody)
 * [`body.extensions`](#requestbodyextensions)
 
-Note: Be particularly careful when interacting with headers in a response context. For router_service(), supergraph_service() and execution_service(), response headers only exist for the first response in a deferred response stream. You can handle this by making use of the `is_primary()` function which will return true if a response is the first (or primary) response. If you do try to access the headers in a non-primary response, then you'll raise an exception which can be handled like any other rhai exception, but is not so convenient as using the `is_primary()` method.
+### `response.is_primary()`
+
+Be particularly careful when interacting with headers in a response context. For router_service(), supergraph_service() and execution_service(), response headers only exist for the first response in a deferred response stream. You can handle this by making use of the `is_primary()` function which will return true if a response is the first (or primary) response. If you do try to access the headers in a non-primary response, then you'll raise an exception which can be handled like any other rhai exception, but is not so convenient as using the `is_primary()` method.
 
 ```rhai
     if response.is_primary() {

examples/cache-control/rhai/src/cache_control.rhai

@@ -43,13 +43,18 @@ fn subgraph_service(service, subgraph) {
 fn supergraph_service(service) {
   // attach the cache-control header if enough data is available
   service.map_response(|response| {
-    let uncacheable = response.context.cache_control_uncacheable;
-    let max_age = response.context.cache_control_max_age;
-    let scope = response.context.cache_control_scope;
 
-    if uncacheable != true && max_age != () && scope != () {
-      response.headers["cache-control"] = `max-age=${max_age}, ${scope}`;
-    }
+    // Note the check for is_primary() to support deferred response headers
+    // https://www.apollographql.com/docs/router/customizations/rhai-api/#response-interface
+    if response.is_primary() {
+      let uncacheable = response.context.cache_control_uncacheable;
+      let max_age = response.context.cache_control_max_age;
+      let scope = response.context.cache_control_scope;
+
+      if uncacheable != true && max_age != () && scope != () {
+        response.headers["cache-control"] = `max-age=${max_age}, ${scope}`;
+      }
+  }
   });
 }