Mark `type` in `AuthenticationData` as optional (#989)

changelogs/client_server/newsfragments/989.clarification

@@ -0,0 +1 @@
+Clarify that the `type` in user-interactive authentication can be omitted.

data/api/client-server/definitions/auth_data.yaml

@@ -17,7 +17,11 @@ description: |-
 type: object
 properties:
   type:
-    description: The login type that the client is attempting to complete.
+    description: |-
+      The authentication type that the client is attempting to complete.
+      May be omitted if `session` is given, and the client is reissuing a
+      request which it believes has been completed out-of-band (for example,
+      via the [fallback mechanism](#fallback)).
     type: string
   session:
     description: The value of the session key given by the homeserver.
@@ -25,8 +29,6 @@ properties:
 additionalProperties:
   description: Keys dependent on the login type
   type: object
-required:
-  - type
 example:
   type: "example.type.foo"
   session: "xxxxx"