Clarify what "suppressing a pointer event stream" really means

[mustaqahmed]

@egirard pointed out in #457 that the first line of Section 4.1.3.3 (Suppressing a pointer event stream) is misleading without certain PEWG background, so it could easily confuse developers.

To be precise, "a pointer is unlikely to continue to produce events" currently carries the implied meaning that "a pointing device is getting disconnected/isolated from the current web-content", and the spec doesn't say it explicitly.

For example, the "modal dialog" condition there is not even a case where the device is stopping to produce events! The device is only stopping to send events to the current web-content.