Clarify VideoFrame|AudioFrame clone/close/transfer behavior

[chcunningham]

Current wording says this is a "deep copy", but makes a note that authors can implement with reference counting under the hood. This has confused at least one user who expected that the "copy" was fine to hold onto.

The text should formalize the reference counting behavior, as it is observable. Additionally, the text should use said behavior in describing new Serailzable and Transferrable characteristics of VideoFrame (what happens when you send to a worker).

[chcunningham]

Title update to include AudioFrame. The two APIs should match. Working on a PR this week.