Spec does not describe the behaviour of `/event/<event ID>` when that event is not available

**Link to problem area**: https://spec.matrix.org/v1.2/server-server-api/#retrieving-events

**Issue**
An arbitrary string passed as an event ID need not be the event ID of a string known to the server. The event may not exist, at all; or it might exist elsewhere, but not on this server. The spec does not say what should happen in this situation.

**Expected behaviour**
- Specify an HTTP status code and Matrix error code for this case.

Open the retrieving events section linked in the issue and review its existing response and error definitions. Document the HTTP status and Matrix error code for unavailable events; it is done when that case is specified in the section.

Written by the indexing model from the issue text.