[web-animations-1] Clarify what it means to "schedule a task" for pending play and pause tasks

[graouts]

There are two procedures in the spec where we say that a task should be scheduled:

1. in 4.4.10. Playing an animation: "Schedule a task to run as soon as animation is ready."

2. in 4.4.11. Pausing an animation : "Schedule a task to be executed at the first possible moment after the user agent has performed any processing necessary to suspend the playback of animation’s target effect, if any."

I think we could be clearer about when these tasks ought to be performed, possibly as part of the update animations and send events procedure.

[graouts]

@birtles @stephenmcgruer @flackr