Add a note to the federation invite endpoints that invites can be sent twice (#2067)

... as this may be non-obvious when implementing behaviour that is triggered by an incoming invite
event.

See https://github.com/matrix-org/matrix-spec/issues/2062 for more context.

Co-authored-by: Kévin Commaille <76261501+zecakeh@users.noreply.github.com>

changelogs/server_server/newsfragments/2067.clarification

@@ -0,0 +1 @@
+Add a note to the invite endpoints that invites to local users may be received twice over federation if the homeserver is already in the room.

data/api/server-server/invites-v1.yaml

@@ -32,6 +32,10 @@ paths:
         [room version specification](/rooms) for precise event formats. **The request and response
         bodies here describe the common event fields in more detail and may be missing other
         required fields for a PDU.**
+
+        Also note that if the remote homeserver is already in the room, it will receive the
+        invite event twice; once through this endpoint, and again through a [federation
+        transaction](/server-server-api/#transactions).
       operationId: sendInviteV1
       security:
         - signedRequest: []

data/api/server-server/invites-v2.yaml

@@ -36,6 +36,10 @@ paths:
         [room version specification](/rooms) for precise event formats. **The request and response
         bodies here describe the common event fields in more detail and may be missing other
         required fields for a PDU.**
+
+        Also note that if the remote homeserver is already in the room, it will receive the
+        invite event twice; once through this endpoint, and again through a [federation
+        transaction](/server-server-api/#transactions).
       operationId: sendInviteV2
       security:
         - signedRequest: []