make definition of signatures fields more consistent

api/client-server/definitions/device_keys.yaml

@@ -47,16 +47,13 @@ properties:
       "ed25519:JLAFKJWSCS": "lEuiRJBit0IG6nUf5pUzWTUEsRVVe/HJkoKuEww9ULI"
   signatures:
     type: object
+    title: Signatures
     description: |-
        Signatures for the device key object. A map from user ID, to a map from
        ``<algorithm>:<device_id>`` to the signature.
 
        The signature is calculated using the process described at `Signing
        JSON`_.
-    additionalProperties:
-      type: object
-      additionalProperties:
-        type: string
     example:
       "@alice:example.com":
         "ed25519:JLAFKJWSCS": "dSO80A01XiigH3uBiDVx/EjzaoycHcjq9lfQX0uWsqxl2giMIiSPR8a4d291W1ihKJL/a+myXS367WT6NAIcBA"

api/server-server/definitions/keys.yaml

@@ -75,18 +75,12 @@ properties:
       required: ["expired_ts", "key"]
   signatures:
     type: object
-    description: Digital signatures for this object signed using the ``verify_keys``.
-    additionalProperties:
-      type: object
-      title: Signed Server
-      example: {
-        "example.org": {
-          "ad25519:abc123": "VGhpcyBzaG91bGQgYWN0dWFsbHkgYmUgYSBzaWduYXR1cmU"
-        }
-      }
-      additionalProperties:
-        type: string
-        name: Encoded Signature Verification Key
+    description: |-
+      Digital signatures for this object signed using the ``verify_keys``.
+
+      The signature is calculated using the process described at `Signing
+      JSON`_.
+    title: Signatures
   valid_until_ts:
     type: integer
     format: int64

api/server-server/third_party_invite.yaml

@@ -102,12 +102,12 @@ paths:
                         properties:
                           signatures:
                             type: object
-                            description: The server signatures for this event.
-                            additionalProperties:
-                              type: object
-                              title: Server Signatures
-                              additionalProperties:
-                                type: string
+                            title: Signatures
+                            description: |-
+                              The server signatures for this event.
+
+                              The signature is calculated using the process
+                              described at `Signing JSON`_.
                             example: {
                               "magic.forest": {
                                 "ed25519:3": "fQpGIW1Snz+pwLZu6sTy2aHy/DYWWTspTJRPyNp0PKkymfIsNffysMl6ObMMFdIJhk6g6pwlIqZ54rxo8SLmAg"

api/server-server/user_keys.yaml

@@ -88,10 +88,12 @@ paths:
                             description: The key, encoded using unpadded base64.
                           signatures:
                             type: object
+                            title: Signatures
                             description: |-
-                              Signature for the device. Mapped from user ID to signature object.
-                            additionalProperties:
-                              type: string
+                              Signature of the key object.
+
+                              The signature is calculated using the process described at `Signing
+                              JSON`_.
                         required: ['key', 'signatures']
                 example: {
                   "@alice:example.com": {