Added clarity to Facebook App Events context.device.type accepted values (#2489)

Author: markzegarelli

src/connections/destinations/catalog/facebook-app-events/index.md

@@ -257,6 +257,9 @@ Facebook requires that payloads include the following:
 - `context.device.type`
 - `context.os.version`
 
+> info ""
+> The value of `context.device.type` must be either `ios` or `android`.
+
 For example:
 
 ```json