Clarify wording on notification queue draining

Clarify wording on notification queue draining

While it's true that the speakers of the sessions will be attendees
of the conference, they probably won't be at this point. I had to
read code to ensure myself this wasn't sending a notification to
all registered attendees of the conference (which it doesn't), so
to spare myself the same excercise the next time reword the text
to make it clear.