Clarify whether QueuedEntries should be cleared on unobserve/disconnect

[alijuma]

When an observation target is unobserved, Chrome clears out any queued entries for that target. Similarly, when disconnect is called, Chrome clears out all of the observer's queued entries.

However, the spec is silent about this, and neither Firefox nor Safari clear queued entries in these situations.

The spec should be explicit about this one way or the other, so then the non-conforming implementation(s) can be updated to match.