Clarify the auth_chain and state for send_join

Fixes https://github.com/matrix-org/matrix-doc/issues/2467
Fixes https://github.com/matrix-org/matrix-doc/issues/2464

This is based on looking at Synapse, not actually testing it. Within the send_join handling functions, an "event context" is acquired [here](d14c4d6b6d/synapse/handlers/federation.py (L1472)) which then later uses the previous state (populated [here](d14c4d6b6d/synapse/state/__init__.py (L286-L289)) in the event context) [here](d14c4d6b6d/synapse/handlers/federation.py (L1494-L1501)) to return a response. Within the event context, the previous state IDs are populated [here](d14c4d6b6d/synapse/state/__init__.py (L351)).

api/server-server/definitions/send_join_response.yaml

@@ -22,9 +22,10 @@ properties:
   auth_chain:
     type: array
     description: |-
-      The auth chain. Note that events have a different format depending on
-      the room version - check the `room version specification`_ for precise
-      event formats.
+      The auth chain for the entire current room state prior to the join event.
+
+      Note that events have a different format depending on the room version - check the
+      `room version specification`_ for precise event formats.
     items:
       type: object
       title: PDU
@@ -39,8 +40,10 @@ properties:
   state:
     type: array
     description: |-
-      The room state. The event format varies depending on the room version -
-      check the `room version specification`_ for precise event formats.
+      The resolved current room state prior to the join event.
+
+      The event format varies depending on the room version - check the `room version specification`_
+      for precise event formats.
     items:
       type: object
       title: PDU

changelogs/server_server/newsfragments/2575.clarification

@@ -0,0 +1 @@
+Clarify the state/auth chain requirements for ``/send_join``.