Also add M_THREEPID_IN_USE

which was always used and is specified in the IS API, but not in the
C/S API. We decided this was well-specced enough that it didn't need
its own MSC.

content/client-server-api/_index.md

@@ -221,6 +221,10 @@ See the [Server Notices](#server-notices) module for more information.
 `M_THREEPID_MEDIUM_NOT_SUPPORTED`
 The homeserver does not support adding a third party identifier of the given medium.
 
+`M_THREEPID_IN_USE`
+The third party identifier specified by the client is not acceptable because it is
+already in use in some way.
+
 #### Rate limiting
 
 Homeservers SHOULD implement rate limiting to reduce the risk of being

data/api/client-server/administrative_contact.yaml

@@ -489,6 +489,8 @@ paths:
           description: |-
             The third-party identifier is already in use on the homeserver, or
             the request was invalid. Error codes that can be returned are:
+              * `M_THREEPID_IN_USE`: The email supplied cannot be bound because is is already
+                associated with a different Matrix ID.
               * `M_SERVER_NOT_TRUSTED`: The server does not trust/support the identity server
                 provided in the request.
               * `M_THREEPID_MEDIUM_NOT_SUPPORTED`: The homeserver does not support adding email addresses.
@@ -550,6 +552,8 @@ paths:
           description: |-
             The third-party identifier is already in use on the homeserver, or
             the request was invalid. Error codes that can be returned are:
+              * `M_THREEPID_IN_USE`: The phone number supplied cannot be bound because is is already
+                associated with a different Matrix ID.
               * `M_SERVER_NOT_TRUSTED`: The server does not trust/support the identity server
               * `M_THREEPID_MEDIUM_NOT_SUPPORTED`: The homeserver does not support adding phone numbers.
               * `M_INVALID_PARAM`: The phone number given was not valid.