Undocumented fiber.stack_size INI directive

[Girgias]

The fiber.stack_size INI directive is not documented.
Found while trying to add a link to it in #1799

[cmb69]

Not a documentation issue, but apparently fiber.stack_size is not part of the default php.ini files of php-src.

[vrana]

Documented now at https://www.php.net/info.configuration#ini.fiber.stack-size.