Added safeAreaViewProps documentation

Author: ArsenyYankovsky

docs/stack-navigator.md

@@ -113,6 +113,10 @@ React Element or Component to display on the left side of the header. When a com
 
 Style object for the header
 
+#### `safeAreaViewProps`
+
+Allow to pass props to internal SafeAreaView used in the header. Currently the main use case is to change `forceInset` property.
+
 #### `headerTitleStyle`
 
 Style object for the title component