Specify that the /openid/userinfo return value must be validated (#2288)

changelogs/server_server/newsfragments/2288.clarification

@@ -0,0 +1 @@
+Specify that callers of `/_matrix/federation/v1/openid/userinfo` must validate the returned user ID.

data/api/server-server/openid.yaml

@@ -43,7 +43,12 @@ paths:
                 properties:
                   sub:
                     type: string
-                    description: The Matrix User ID who generated the token.
+                    description: |
+                      The Matrix User ID who generated the token.
+
+                      The caller MUST validate that the returned user ID is on the server they
+                      called (i.e. if you make a request to example.com and it returns
+                      `@alice:matrix.org`, the result is invalid).
                     example: "@alice:example.com"
                 required:
                   - sub