doc: Event RunType

[lambdai]

Signed-off-by: Yuchen Dai silentdai@gmail.com

Description:
See http://www.wangafu.net/~nickm/libevent-book/Ref3_eventloop.html
Pseudocode
while (any events are registered with the loop,
or EVLOOP_NO_EXIT_ON_EMPTY was set) {

if (EVLOOP_NONBLOCK was set, or any events are already active)
    If any registered events have triggered, mark them active.
else
    Wait until at least one event has triggered, and mark it active.

...
}

Risk Level: LOW
Testing:
Docs Changes:
Release Notes:

[mattklein123]

Thanks for the contribution, but from an interface perspective we are mostly trying to the hide the libevent details and I think the existing text is probably fine? I'm not sure it's really worth updating.

[lambdai]

Our code base is relying on the described behavior. See below.
Also original comment bite me when I am writing integration test.

      // Must create a keepalive timer to keep the dispatcher from exiting.
      std::chrono::milliseconds time_interval(500);
      keepalive_timer_ = dispatcher_->createTimer(...);
      dispatcher_->run(Dispatcher::RunType::Block);