docs(config): document runtime_fallback boolean shorthand

Add simple boolean configuration examples for runtime_fallback:
- true/false for quick enable/disable
- Object format for advanced configuration

docs/reference/configuration.md

@@ -529,6 +529,13 @@ Configure Language Server Protocol integration:
 
 Auto-switches to backup models on API errors.
 
+**Simple configuration** (enable/disable with defaults):
+```json
+{ "runtime_fallback": true }
+{ "runtime_fallback": false }
+```
+
+**Advanced configuration** (full control):
 ```json
 {
   "runtime_fallback": {