Incorrect return value documented for set_time_limit.

Documented as void, but the function has always returned a boolean true/false.
See source code for reference http://lxr.php.net/xref/PHP_5_5/main/main.c#1280

git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@334526 c90b9560-bf6c-de11-be94-00142212c4b1