Clarify cross-element uniqueness requirements of callbackId

[markafoltz]

From #83 (comment)

@avayvod: I think this example implies the callback ids must be unique across all the media elements within a browser session which is not true. They only must be unique for each media element alone. An example is a counter in the media element's scope, that's set to 0 when the media element is created and is incremented each time its value is returned by watchAvailability() for this particular element as the callback id.

@mfoltzgoogle: That seems inconvenient if a page is watching availability on multiple elements and wants to keep the callbacks in a Map or otherwise track the set of callbacks by id.

@avayvod: Fair point. It might be more likely having one callback per many elements compared to many callbacks for a single element. The spec doesn't guarantee any cross-element uniqueness of the ids at the moment though.

CC @mounirlamouri @jernoble

[markafoltz]

Closing as I believe this is addressed by the PR.