Document `validated_at` and `added_at` on GET /3pid

Fixes https://github.com/matrix-org/matrix-doc/issues/661

api/client-server/administrative_contact.yaml

@@ -50,7 +50,9 @@ paths:
               "threepids": [
                 {
                   "medium": "email",
-                    "address": "monkey@banana.island"
+                  "address": "monkey@banana.island",
+                  "validated_at": 1535176800000,
+                  "added_at": 1535336848756
                 }
               ]
             }
@@ -70,6 +72,19 @@ paths:
                     address:
                       type: string
                       description: The third party identifier address.
+                    validated_at:
+                      type: integer
+                      format: int64
+                      description: |-
+                        The timestamp, in milliseconds, when the identifier was
+                        validated by the identity service.
+                    added_at:
+                      type: integer
+                      format: int64
+                      description:
+                        The timestamp, in milliseconds, when the homeserver
+                        associated the third party identifier with the user.
+                  required: ['medium', 'address', 'validated_at', 'added_at']
       tags:
         - User data
     post: