Clarify (room_id, event_id) global uniqueness (#13701)

Summarized from @richvdh's reply at #13589 (comment)

changelog.d/13701.doc

@@ -0,0 +1 @@
+Clarify `(room_id, event_id)` global uniqueness and how we should scope our database schemas.

docs/development/database_schema.md

@@ -191,3 +191,27 @@ There are three separate aspects to this:
    flavour will be accepted by SQLite 3.22, but will give a column whose
    default value is the **string** `"FALSE"` - which, when cast back to a boolean
    in Python, evaluates to `True`.
+
+
+## `event_id` global uniqueness
+
+In room versions `1` and `2` it's possible to end up with two events with the
+same `event_id` (in the same or different rooms). After room version `3`, that
+can only happen with a hash collision, which we basically hope will never
+happen.
+
+There are several places in Synapse and even Matrix APIs like [`GET
+/_matrix/federation/v1/event/{eventId}`](https://spec.matrix.org/v1.1/server-server-api/#get_matrixfederationv1eventeventid)
+where we assume that event IDs are globally unique.
+
+But hash collisions are still possible, and by treating event IDs as room
+scoped, we can reduce the possibility of a hash collision. When scoping
+`event_id` in the database schema, it should be also accompanied by `room_id`
+(`PRIMARY KEY (room_id, event_id)`) and lookups should be done through the pair
+`(room_id, event_id)`.
+
+There has been a lot of debate on this in places like
+https://github.com/matrix-org/matrix-spec-proposals/issues/2779 and
+[MSC2848](https://github.com/matrix-org/matrix-spec-proposals/pull/2848) which
+has no resolution yet (as of 2022-09-01).
+