The handling of the 3rd parameter for assert callbacks set via `assert_options`
changed with the implementation of the expectations RFC, and changed again in
PHP 8. Update the documentation accordingly.
This one seems to be a cause for confusion; witness Bug [#79602]
(https://bugs.php.net/bug.php?id=79602).
Co-authored-by: Christoph M. Becker <cmbecker69@gmx.de>
Closes GH-120.