Add more detail to interstitials docs.
Add more detail to interstitials docs.

8tracks/appnexus-mobile-sdk-ios,appnexus/mobile-sdk-ios,appnexus/mobile-sdk-ios,8tracks/appnexus-mobile-sdk-ios,8tracks/appnexus-mobile-sdk-ios,appnexus/mobile-sdk-ios,appnexus/mobile-sdk-ios,appnexus/mobile-sdk-ios,8tracks/appnexus-mobile-sdk-ios,8tracks/appnexus-mobile-sdk-ios,appnexus/mobile-sdk-ios