clarify that the note is about Backend constructors and not the allowDynamicBackends function (#664)

Author: JakeChampion

documentation/docs/fastly:backend/Backend/Backend.mdx

@@ -21,7 +21,7 @@ import { allowDynamicBackends } from "fastly:experimental";
 allowDynamicBackends(true);
 ```
 
-**Note**: Can only be used when processing requests, not during build-time initialization.
+**Note**: Backend constructors can only be used when processing requests, not during build-time initialization.
 
 ## Syntax
 
@@ -224,4 +224,4 @@ async function app() {
 addEventListener("fetch", event => event.respondWith(app(event)));
 ```
 
-</Fiddle>
+</Fiddle>