docs: add note about maybe-dangerous use of currentActivity

Hopefully no one will ever read it or care!
But if there are crashes, our current understanding and reasoning is now noted for future maintainer use

android/src/main/java/io/invertase/googlemobileads/ReactNativeGoogleMobileAdsModule.kt

@@ -95,6 +95,11 @@ class ReactNativeGoogleMobileAdsModule(
   @ReactMethod
   fun initialize(promise: Promise) {
     MobileAds.initialize(
+      // in react-native, the Activity instance *may* go away, becoming null after initialize
+      // it is not clear if that can happen here without an initialize necessarily following the Activity lifecycle
+      // it is not clear if that will cause problems even if it happens, but users that have widely deployed this
+      // with the use of currentActivity have not seen problems
+      // reference if it needs attention: https://github.com/invertase/react-native-google-mobile-ads/pull/664
       currentActivity ?: reactApplicationContext,
       OnInitializationCompleteListener { initializationStatus ->
         val result = Arguments.createArray()