Update Native Stack search bar props

[kligarski]

Supersedes #1440 (due to conflicts I decided to reapply changes manually + I added information about autoCapitalize fix).

Updates search bar props in native stack docs.

Related PRs in react-native-screens:

• feat(iOS 26, Stack v4): support for new search bar placements, integration with toolbar software-mansion/react-native-screens#3168
• feat!(iOS, Stack v4): change search bar related prop defaults software-mansion/react-native-screens#3186
• fix(iOS, Stack v4): ensure consistent defaults for obscureBackground and hideNavigationBar software-mansion/react-native-screens#3199
• feat(iOS): add support for using native behavior in hideNavigationBar, obscureBackground props in SearchBar software-mansion/react-native-screens#3211
• fix(iOS, Stack v4): add systemDefault option to autoCapitalize prop in SearchBar software-mansion/react-native-screens#3251

Changes

• remove outdated platform support information in headerSearchBarOptions description (Android also supports search bar)
• update descriptions for hideNavigationBar, obscureBackground to account for handling native behavior
• add placement, allowToolbarIntegration props
• add information about deprecation of cancelButtonText starting from iOS 26
• update information about default value of autoCapitalize

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