docs: improve documentation a bit

Signed-off-by: Hellgren Heikki <heikki.hellgren@op.fi>

docs/backend-system/core-services/http-router.md

@@ -81,7 +81,7 @@ backend:
   rateLimit: true
 ```
 
-You can additionally configure the rate limiting parameters also by plugin:
+You can additionally configure the rate limiting parameters, also by plugin:
 
 ```yaml
 backend:
@@ -112,6 +112,17 @@ backend:
       connection: redis://127.0.0.1:16379
 ```
 
+If your instance is working behind a proxy, you have to configure the backend to trust the proxy
+for the rate limiting being able to distinguish clients.
+
+```yaml
+backend:
+  trustProxy: true
+```
+
+For more information about the trust proxy configuration and available options,
+please refer to [express documentation](https://expressjs.com/en/guide/behind-proxies.html).
+
 ## Configuring the service
 
 For more advanced customization, there are several APIs from the `@backstage/backend-defaults/httpRouter` package that allow you to customize the implementation of the config service. The default implementation uses all of the middleware exported from `@backstage/backend-defaults/httpRouter`, including `createLifecycleMiddleware`, `createAuthIntegrationRouter`, `createCredentialsBarrier` and `createCookieAuthRefreshMiddleware`. You can use these to create your own `httpRouter` service implementation, for example - here's how you would add a custom health check route to all plugins:
@@ -122,6 +133,7 @@ import {
   createCookieAuthRefreshMiddleware,
   createCredentialsBarrier,
   createAuthIntegrationRouter,
+  createRateLimitMiddleware,
 } from '@backstage/backend-defaults/httpRouter';
 import { createServiceFactory } from '@backstage/backend-plugin-api';
 
@@ -149,6 +161,11 @@ backend.add(
     }) {
       const router = PromiseRouter();
 
+      // Optional rate limiting middleware
+      router.use(
+        createRateLimitMiddleware({ pluginId: plugin.getId(), config }),
+      );
+
       rootHttpRouter.use(`/api/${plugin.getId()}`, router);
 
       const credentialsBarrier = createCredentialsBarrier({

docs/backend-system/core-services/root-http-router.md

@@ -40,6 +40,10 @@ createBackendPlugin({
 });
 ```
 
+## Rate limiting
+
+Please refer to the [HTTP Router documentation](./http-router.md#rate-limiting).
+
 ## Configuring the service
 
 ### Via `app-config.yaml`