Adding WA.onInit method to wait for API startup.

docs/maps/api-start.md

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+{.section-title.accent.text-primary}
+# API start functions Reference
+
+### Waiting for WorkAdventure API to be available
+
+When your script / iFrame loads WorkAdventure, it takes a few milliseconds for your script / iFrame to exchange
+data with WorkAdventure. You should wait for the WorkAdventure API to be fully ready using the `WA.onInit()` method.
+
+```
+WA.onInit(): Promise<void>
+```
+
+Some properties (like the current user name, or the room ID) are not available until `WA.onInit` has completed.
+
+Example:
+
+```typescript
+WA.onInit().then(() => {
+    console.log('Current player name: ', WA.player.name);
+});
+```
+
+Or the same code, using await/async:
+
+```typescript
+(async () => {
+    await WA.onInit();
+    console.log('Current player name: ', WA.player.name);
+})();
+```