arch: elaborate on firehose/sync mechanism a bit

[bnewbold]

The sync mechanism is (or will be) more completely described in the repo sync doc. But we got feedback that more detail in the arch document would be helpful.

The closest current description:

Updates to repositories, identities, and account status are all broadcast publicly over a "firehose" WebSocket, which any party can subscribe to.

Maybe a sub-section under 3. Data Repositories?