MediaSessionConnector: Document how to provide metadata asynchronously

[ojw28]

To provide metadata asynchronously, you can:

1. Return placeholder metadata from MediaMetadataProvider.getMetadata
2. Kick off the async operation, which will to place the metadata into some kind of cache
3. Call invalidateMediaSessionMetadata when the async operation completes
4. Return real metadata from MediaMetadataProvider.getMetadata, assuming the player is still playing the same media item (probably implemented by having the cache be keyed on an identifier)

We should more clearly document this somehow. The best place would be to document this on getMetadata directly. If we write a web page about MediaSessionConnector, it would be good to document there also.