Clarify if notifications are cleared by stream ordering or topological ordering

**Link to problem area**: https://spec.matrix.org/latest/client-server-api/#receiving-notifications

**Issue**:

It is not stated clearly which notifications get cleared when a read receipt is received, currently the spec reads:

> When the user updates their read receipt (either by using the API or by sending an event), notifications prior to and including that event MUST be marked as read.

It is unclear if "prior to" refers to stream ordering or topological ordering. Synapse clears this based on stream ordering.

(Note that because of the inconsistency in `/sync` and `/messages` either could give odd results to a user.)

See conversation around https://matrix.to/#/!NasysSDfxKxZBzJJoE:matrix.org/$sb6NPRyuiu4wN7egyY8SJ6YoubvQqZvJSOlMFsTzVSE?via=matrix.org&via=libera.chat&via=element.io

Start with the receiving notifications section of the Matrix client-server API specification and compare its read-receipt wording with Synapse's stream-ordering behavior. Review the linked conversation and determine whether stream ordering or topological ordering is intended; done means the specification explicitly identifies the ordering used to clear notifications.

Written by the indexing model from the issue text.