Update docs for animationDuration in 7.x (react-navigation#1436)

Author: kmichalikk

versioned_docs/version-7.x/native-stack-navigator.md

@@ -604,7 +604,7 @@ Only supported on iOS.
 
 Changes the duration (in milliseconds) of `slide_from_bottom`, `fade_from_bottom`, `fade` and `simple_push` transitions on iOS. Defaults to `350`.
 
-The duration of `default` and `flip` transitions isn't customizable.
+For screens with `default` and `flip` transitions, and, as of now, for screens with `presentation` set to `modal`, `formSheet`, `pageSheet` (regardless of transition), the duration isn't customizable.
 
 Only supported on iOS.