Patch call_user_function.xml

Hello, This is my first patch for the PHP documentation. I've just started writing extensions for PHP and found that documentation is quite limited. So I've started with describing functions as soon as I got them working. This first one is for the call_user_function ZEND API docs. If these kind of documentations look useful, just let me know and there'll probably be more. Ron PS. For now I'm not on the phpdoc list, I'll join if you decide my patches are worth anything. But until then, please just cc me in any reply. PS. make test worked find with the patch, however I couldn't preview the file since the tip in the doc howto chapter 16 doesn't seem to work.