Update index.d.ts

Author: Ali Abdelfattah

index.d.ts

@@ -558,7 +558,8 @@ export namespace APM {
    */
   function setAppLaunchEnabled(isEnabled: boolean): void;
   /**
-   * Ends app launch
+   * Ends the current session’s App Launch. Calling this API is optional, App Launches will still be captured and ended automatically by the SDK;
+   * this API just allows you to change when an App Launch actually ends.
    */
   function endAppLaunch(): void;
   /**