docs(functions): show how to use region-specific function (#6993)

Co-authored-by: Mike Hardy <[email protected]>

Author: Bilal-Abdeen

docs/functions/usage/index.md

@@ -103,3 +103,17 @@ function App() {
   // ...
 }
 ```
+
+## Region-specific Functions
+
+If you need to deploy Functions in a region other than the default one, modified statements need to be used.
+
+```javascript
+import { firebase } from '@react-native-firebase/functions';
+
+// Emulator statement
+firebase.app().functions('region_name').useEmulator('localhost', 5001);
+
+// function calling statement
+firebase.app().functions('region_name').httpsCallable('listProducts')({ abc: 123 }).then();
+```