Add version 4 rooms to the spec

As per [MSC1884](https://github.com/matrix-org/matrix-doc/pull/1884) and [MSC2002](https://github.com/matrix-org/matrix-doc/pull/2002).

No known changes since the proposals were accepted.

Due to being in the area: This fixes https://github.com/matrix-org/matrix-doc/issues/1863

api/server-server/definitions/pdu_v3.yaml

@@ -20,6 +20,10 @@ allOf:
   - $ref: "unsigned_pdu_base.yaml"
   - type: object
     properties:
+      redacts:
+        type: string
+        description: For redaction events, the ID of the event being redacted.
+        example: "$def/456+oldevent"
       auth_events:
         type: array
         items:

api/server-server/definitions/pdu_v4.yaml

@@ -0,0 +1,47 @@
+# Copyright 2019 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+type: object
+title: Persistent Data Unit
+description: A persistent data unit (event) for room version 3 and beyond.
+example:
+  $ref: "../examples/pdu_v4.json"
+allOf:
+  - $ref: "pdu_v3.yaml"
+  - type: object
+    properties:
+      redacts:
+        type: string
+        description: For redaction events, the ID of the event being redacted.
+        example: "$def_456-oldevent"
+      auth_events:
+        type: array
+        items:
+          type: string
+          description: Event ID.
+        description: |-
+          Event IDs for the authorization events that would
+          allow this event to be in the room.
+        example: ["$URLsafe-base64EncodedHash", "$Another_Event"]
+      prev_events:
+        type: array
+        items:
+          type: string
+          description: Event ID.
+        description: |-
+          Event IDs for the most recent events in the room
+          that the homeserver was aware of when it made this event.
+        example: ["$URLsafe-base64EncodedHash", "$Another_Event"]
+    required:
+      - auth_events
+      - prev_events

api/server-server/examples/pdu_v3.json

@@ -15,5 +15,6 @@
     "prev_events": [
         "$base64encodedeventid",
         "$adifferenteventid"
-    ]
+    ],
+    "redacts": "$some/old+event"
 }

api/server-server/examples/pdu_v4.json

@@ -0,0 +1,12 @@
+{
+    "$ref": "pdu_v3.json",
+    "auth_events": [
+        "$urlsafe_base64_encoded_eventid",
+        "$a-different-event-id"
+    ],
+    "prev_events": [
+        "$urlsafe_base64_encoded_eventid",
+        "$a-different-event-id"
+    ],
+    "redacts": "$some-old_event"
+}