Deprecate verifications that don't begin with a request.
@@ -528,7 +528,9 @@ method, then the verification should be cancelled with a `code` of
`m.unexpected_message`.
An `m.key.verification.start` message can also be sent independently of any
-request, specifying the verification method to use.
+request, specifying the verification method to use. This behaviour is
+deprecated, and new clients should not begin verifications in this way.
+However, clients should handle such verifications started by other clients.
Individual verification methods may add additional steps, events, and
properties to the verification messages. Event types for methods defined
