Update ios-sdk-integration-gam-original-api.md

doc: update iOS DOC

prebid-mobile/pbm-api/ios/ios-sdk-integration-gam-original-api.md

@@ -43,23 +43,28 @@ This section describes the integration details for different ad formats. In each
 
 ### Impression tracking
 
-The SDK supports the native impression tracking. It triggers the `burl` impression URL. It tracks the visibility of view and trigger the event when the view is on the screen and at least 1x1 px in size.
+In the Bidding Only integration scenario, PUC is responsible for tracking events for banner ads, like `burl`, `imp`, and `win`. The disadvantage of this approach is that PUC doesn't have reliable information about the viewability of the WebView. As a result, impression tracking happens at the rendering stage of the ad. Or, if MRAID is supported, once the `viewableChange` event is fired. It leads to big discrepancies since the "1 pixel in view" requirement is not met.
 
-For activation for the banner ad units you should use `fetchDemand()` with the ad view parameter (f.e. GAMBannerView):
+Starting with version `2.4.0`, Prebid SDK introduced the API to track the viewability of the ad and track impression event, respectively. 
+
+To activate impression tracking for the banner ad unit - use the `activatePrebidImpressionTracker(adView)` method. The `adView` parameter should be an instance of AdManagerAdView:
 
 ```swift
+adUnit.activatePrebidImpressionTracker(adView: gamBanner)
 adUnit.fetchDemand(adObject: gamRequest, adView: gamBanner) { [weak self] resultCode in
     // ...
 }
 ```
 
-For activation for the interstitial ad unit you should set `activatePrebidImpressionTracker` flag:
+For activation for the interstitial ad unit, you should set `activatePrebidImpressionTracker()` flag:
 
 ```swift
 let adUnit = InterstitialAdUnit(configId: CONFIG_ID, minWidthPerc: WIDTH_PERC, minHeightPerc: HEIGTH_PERC)
-adUnit.activatePrebidImpressionTracker = true
+adUnit.activatePrebidImpressionTracker()
 ```
 
+After the invocation of `activatePrebidImpressionTracker(),` the Prebid SDK will start analyzing the View Hierarchy and track the viewability of the ad view. Once the ad view is viewable for the user for at least 1 pixel for 1 second, the SDK will track an impression event for the presented ad. The SDK will stop analyzing the View Hierarchy once the caller object of `activatePrebidImpressionTracker()` is destroyed. 
+
 ## Further Reading
 
 - [Prebid Mobile Overview](/prebid-mobile/prebid-mobile.html)