Add missing onSearchButtonPress option to native-stack-navigator.md

This important option was missing from the documentation. I found it in the underlying library's documentation at

https://github.com/software-mansion/react-native-screens/blob/main/guides/GUIDE_FOR_LIBRARY_AUTHORS.md#screenstackheaderconfig

Author: kellycampbell

versioned_docs/version-6.x/native-stack-navigator.md

@@ -370,6 +370,10 @@ A callback that gets called when the cancel button is pressed.
 
 A callback that gets called when the text changes. It receives the current text value of the search bar.
 
+##### `onSearchButtonPress`
+
+A callback that gets called when the search button is pressed. It receives the current text value of the search bar.
+
 Example:
 
 ```js

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

@@ -370,6 +370,10 @@ A callback that gets called when the cancel button is pressed.
 
 A callback that gets called when the text changes. It receives the current text value of the search bar.
 
+##### `onSearchButtonPress`
+
+A callback that gets called when the search button is pressed. It receives the current text value of the search bar.
+
 Example:
 
 ```js