LGTM. Could use an entry in the UPGRADING file.

Given that this is basically internal documentation, I'm not entirely sure what to write - the functions that only returned true now enforce that?

Something like this will do, like we did for some functions in PHP 8.4:

readline_add_history, readline_clear_history, and readline_callback_handler_install functions
now have a return type of true instead of bool

Something like this will do, like we did for some functions in PHP 8.4:

readline_add_history, readline_clear_history, and readline_callback_handler_install functions
now have a return type of true instead of bool

Done