mainloop: Document need for mainloop lock around pa_mainloop_api_once

mainloop: Document need for mainloop lock around pa_mainloop_api_once

This needs us to expose a bit of implementation detail, but this seems
to be the cleanest way without an API change.

The specific problem is that pa_mainloop_api_once() needs to first
create a defer event and then set its destroy callback. If the defer
event is completed before the callback is set, an assert will be
trigerred.