docs: Update router-context.md with context usage tips (#5540)

Author: johnf

docs/router/framework/react/guide/router-context.md

@@ -42,6 +42,9 @@ const router = createRouter({
 })
 ```
 
+> [!TIP]
+> `MyRouterContext` only needs to contain content that will be passed directly to `createRouter` below. All other context added in `beforeLoad` will be inferred.
+
 ## Passing the initial Router Context
 
 The router context is passed to the router at instantiation time. You can pass the initial router context to the router via the `context` option: