Clarify formats of string types in current account information section

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

data/api/client-server/whoami.yaml

@@ -44,6 +44,8 @@ paths:
                 properties:
                   user_id:
                     type: string
+                    format: mx-user-id
+                    pattern: "^@"
                     description: The user ID that owns the access token.
                   device_id:
                     x-addedInMatrixVersion: "1.1"

data/event-schemas/schema/m.identity_server.yaml

@@ -9,7 +9,10 @@ properties:
     type: object
     properties:
       base_url:
-        type: string
+        anyOf:
+          - type: string
+            format: uri
+          - type: "null"
         description: |-
           The URL of the identity server the user prefers to use, or `null`
           if the user does not want to use an identity server. This value is