Move prev_content to unsigned (#3524)

data/event-schemas/schema/core-event-schema/sync_state_event.yaml

@@ -19,11 +19,6 @@ allOf:
 description: In addition to the Room Event fields, State Events have the following
   additional fields.
 properties:
-  prev_content:
-    description: Optional. The previous `content` for this event. If there is no
-      previous content, this key will be missing.
-    title: EventContent
-    type: object
   state_key:
     description: A unique key which defines the overwriting semantics for this piece
       of room state. This value is often a zero-length string. The presence of this

data/event-schemas/schema/core-event-schema/unsigned_prop.yaml

@@ -32,3 +32,17 @@ properties:
       `PUT /_matrix/client/r0/rooms/{roomId}/send/{eventType}/{txnId}`,
       if the client being given the event is the same one which sent it.
     type: string
+  prev_content:
+    description: |
+      The previous `content` for this event. This field is generated
+      by the local homeserver, and is only returned if the event is a state event,
+      and the client has permission to see the previous content.
+    x-changedInMatrixVersion:
+      1.2: |
+        Previously, this field was specified at the top level of returned
+        events rather than in `unsigned` (with the exception of the [`GET
+        .../notifications`](/client-server-api/#get_matrixclientv3notifications)
+        endpoint), though in practice no known server implementations honoured
+        this.
+    title: EventContent
+    type: object