Don't explain how the IS might accept a 3pid

The IS is bound to it's own set of specifications, and if the client chose this API then they should be aware of the risks but not necessarily arbitrary alternatives.

api/client-server/administrative_contact.yaml

@@ -218,8 +218,7 @@ paths:
         403:
           description: |-
             The homeserver does not permit the user from having the third party
-            identifier as a contact option. This does not prevent the identity
-            service from binding the third party identifier, however.
+            identifier as a contact option.
           schema:
             $ref: "definitions/errors/error.yaml"
           examples:
@@ -273,8 +272,7 @@ paths:
         403:
           description: |-
             The homeserver does not permit the user from having the third party
-            identifier as a contact option. This does not prevent the identity
-            service from binding the third party identifier, however.
+            identifier as a contact option.
           schema:
             $ref: "definitions/errors/error.yaml"
           examples: