Switch from additionalProperties: {} to using a description

changelogs/client_server/newsfragments/1909.clarification

@@ -1 +1 @@
-The `UserIdentifier` object in `POST /_matrix/client/v3/login` can contain additional properties.
+The `UserIdentifier` object in `POST /_matrix/client/v3/login` contains additional properties that depend on the identification type.

changelogs/internal/newsfragments/1909.clarification

@@ -1 +0,0 @@
-Enforce `additionalProperties: {}` instead of the equivalent `additionalProperties: true` for easier processing.

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: {}
+additionalProperties:
+  description: Keys dependent on the identification type

layouts/partials/openapi/render-object-table.html

@@ -53,11 +53,6 @@
 
     {{ end }}
 
-    {{/* Enforce `additionalProperties: {}` instead of the equivalent `additionalProperties: true` for easier processing */}}
-    {{ if eq .additionalProperties true }}
-      {{ errorf "Use `additionalProperties: {}` instead of `additionalProperties: true`" }}
-    {{ end }}
-
     {{/*
       If the object has additional properties *as well as* regular properties, we add a special row to the table.
 
@@ -72,6 +67,9 @@
 
       TODO: support `patternProperties` here.
     */}}
+    {{ if eq .additionalProperties true }}
+      {{ errorf "`additionalProperties: true` is de facto the default for Matrix and doesn't provide any information" }}
+    {{ end }}
     {{ if reflect.IsMap .additionalProperties }}
 
  <tr>