[integration] clarify accessability differences between local and cloud API

Author: capcom6

docs/integration/api.md

@@ -5,3 +5,11 @@ The SMS Gateway for Android™ provides a robust API that allows you to send SMS
 ## API Specification 📄
 
 You can find the OpenAPI specification for our API at the following link: [OpenAPI Specification](https://capcom6.github.io/android-sms-gateway/). This specification includes detailed information about the available endpoints, request/response structure, and more.
+
+!!! note "Local vs Cloud API Access"
+    While both API endpoints use the same authentication method and request format, there are critical differences in accessibility:
+    
+    - **Local Server API**: Only accessible within your local network (same Wi-Fi/Ethernet)
+    - **Cloud API**: Accessible from anywhere on the internet
+    
+    External services like Google Apps Script, AWS Lambda, or other cloud functions **cannot** directly access Local Server API endpoints due to network constraints.