Clarify that the `User identifier` object in `POST /_matrix/client/v3/login` contains additional properties that depend on the identification type (#1909)

Signed-off-by: Johannes Marbach <n0-0ne+github@mailbox.org>

changelogs/client_server/newsfragments/1909.clarification

@@ -0,0 +1 @@
+The `User identifier` object in `POST /_matrix/client/v3/login` contains additional properties that depend on the identification type.

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

@@ -18,7 +18,10 @@ type: object
 properties:
   type:
     type: string
-    description: The type of identification.  See [Identifier types](/client-server-api/#identifier-types) for supported values and additional property descriptions.
+    description: |-
+      The type of identification. See [Identifier types](/client-server-api/#identifier-types)
+      for supported values and additional property descriptions.
 required:
   - type
-additionalProperties: true
+additionalProperties:
+  description: Keys dependent on the identification type.