[Sync-EN] Optimize the Swoole event documentation

[lacatoire]

Syncs the reference/swoole/ event pages with php/doc-en#4774.

The Swoole\Event pages were essentially empty stubs: the class intro, every method description and almost every parameter description held an empty paragraph. They are now translated in full:

• swoole.event.xml: the intro explains that the extension exposes the underlying epoll/kqueue/poll/select loop and lets sockets from other extensions join it, with the note that the module is low-level. Two new sections are added: event priority, and the four accepted socket types (fd, stream resource, socket resource, object).
• Swoole\Event::add(), del(), set(), wait(), write(), defer(): full descriptions, parameters, return values, examples and the accompanying notes and warnings, including that a socket cannot be added twice, that del() must be called before closing a socket to avoid leaks, the set() callback replacement rules, and the write() buffer overflow behaviour and SSL restriction.
• Adds Swoole\Event::cycle(), Swoole\Event::dispatch() and Swoole\Event::isset(), translating the three new pages.
• Removes swoole/event/exit.xml, as upstream; the procedural swoole_event_exit() page is untouched.

The add() and set() synopses also gain the correct types: sock is mixed, not int/string, and the fourth parameter is flags of type int, not events of type string.

EN-Revision bumped to 282797eedc7c7bca6a6f55a9aad32b135feb3320 on the affected files.

Fixes: #1618