Clarify "accessible" and "asynchonously"

[mwatson2]

The specification describes the MediaKeySession lifecycle in terms of whether it is "accessible", based on an original discussion and proposal here: https://www.w3.org/Bugs/Public/show_bug.cgi?id=24771

It may not be clear to readers what this means. I suggest an additional note, after the 4th paragraph of Section 6, as follows:

    Accessible means that the page maintains a reference to the object, or could obtain one from another object to which it has access (in this case from a Media Element). Therefore, the above requirement means that a <a>MediaKeySession</a> remains active and continues to receive events provided the page maintains a reference to the associated <a>MediaKeys</a> object or a Media Element to which it is attached. If that is not the case, the <a>MediaKeySession</a> could never be used and will not receive further events.

Additionally, this section uses the term "asynchronously" when it should say "in parallel".