Server-Server API does not specify that sending servers need to be able to give `prev_events` for the events that they send

**Link to problem area**: https://spec.matrix.org/v1.3/server-server-api/#checks-performed-on-receipt-of-a-pdu

Link to relevant Synapse code: https://github.com/matrix-org/synapse/blob/0bf180cbb43f6e3d489ddaa5d66226b6daeea027/synapse/handlers/federation_event.py#L307-L315

**Issue**

Synapse requires that a remote homeserver which is sending an event is able to produce, on demand, the `prev_events` for that event via `/get_missing_events`. This does not appear to be made clear in the specification, which I think could be troublesome for interoperability.
It also just seems like something that is worth noting; particularly when looking to implement faster room joins, this is something that has come up.

Read the linked Server-Server API section, then compare its receipt checks with the linked Synapse federation_event.py code around lines 307-315. Update the specification to state that sending servers must be able to provide the prev_events for events they send, and confirm the wording clearly addresses interoperability.

Written by the indexing model from the issue text.