Document 6 ext/libevent functions as partial fix for bug 67420

The remaining missing function event_timer_pending() will be documented when the exact purpose and API has been agreed with the package maintainers, at present it appears to be incorrectly implemented.

git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@333778 c90b9560-bf6c-de11-be94-00142212c4b1