docs(config options): example to limit registryAliases to one manager (#30038)

Author: HonkingGoose

docs/usage/configuration-options.md

@@ -3722,7 +3722,7 @@ This feature works with the following managers:
 - [`terraform`](modules/manager/terraform/index.md)
 - [`woodpecker`](modules/manager/woodpecker/index.md)
 
-```json
+```json title="Setting generic aliases"
 {
   "registryAliases": {
     "jfrogecosystem": "some.jfrog.mirror",
@@ -3731,6 +3731,17 @@ This feature works with the following managers:
 }
 ```
 
+```json title="Setting aliases for a specific Renovate manager only"
+{
+  "gitlabci": {
+    "registryAliases": {
+      "$HARBOR_HOST/$HARBOR_PROJECT": "registry.example.com/proxy",
+      "$HARBOR_HOST/tools": "registry.example.com/tools"
+    }
+  }
+}
+```
+
 ## registryUrls
 
 Usually Renovate is able to either (a) use the default registries for a datasource, or (b) automatically detect during the manager extract phase which custom registries are in use.