Clarifies callbackId requirements.

[markafoltz]

PTAL @avayvod @mounirlamouri

Addresses:

• Issue [Privacy] Clarify steps to generate callbackId #81: [Privacy] Clarify steps to generate callbackId
• Issue Clarify cross-element uniqueness requirements of callbackId #85: Clarify cross-element uniqueness requirements of callbackId

This asserts that the callbackId is unique among all ids assigned for a given browsing context (frame), ensuring that there will be no duplicates in the set of availability callbacks for a given media element, and that Web developers can aggregate callbacks across media elements without worrying about duplicate ids.

Note that calling watchAvailability twice with the same callback object is legal:

1. It will generate two distinct ids.
2. It will be invoked twice when availability changes.
3. It will need to be canceled twice (once with each id).