Document restore_exception_handler behavior change in PHP 8.3.5

[lacatoire]

Document the current exception handler stack behavior, which changed in two steps.

Since PHP 8.3.0 the engine unsets the active handler before invoking it, so no handler is active while one runs. Since PHP 8.3.5 it also pushes the handler it is about to invoke onto the handler stack, and re-installs it once it returns, but only if no handler is active at that point. Consequences documented on both pages:

• restore_exception_handler() called from within a handler re-installs the handler that is currently running, so restoring the handler that was active before it requires two calls;
• as soon as the handler calls set_exception_handler() or restore_exception_handler(), the automatic restoration is skipped and the handler is left in charge of the stack.

Verified against the official CLI images: 7.4.33, 8.1.34, 8.2.28 and 8.3.4 show the previous behavior, 8.3.6, 8.4.24 and 8.5.4 the new one. The change is not present in the 8.2 branch.

Also unwraps the <simplelist> of both seealso sections from its <para>.

Sources

• Active handler unset while it runs, released in PHP 8.3.0: php/php-src@b3e33be
• Handler pushed onto the stack and restored after running: php/php-src@3301d96 (GH-13446, Restore exception handler after running it php-src#13686)
• Released in PHP 8.3.5: NEWS entry Fixed bug GH-13446 (Restore exception handler after it finishes).

Fixes #5102