Document how oEmbed works

[Half-Shot]

Description:

Specifically:

1. Synapse bundles an official set of providers, and allows you to provide your own.
2. If there is a HIT, then Synapse can quickly do a oEmbed URL preview of the URL you sent.
3. If there is a MISS, we still perform discovery on the URL.
4. This occurs regardless of whether you configure oembed in your config.