Clarification for `/event_auth` (#3583)

The description on this endpoint implied it returned the auth chain for the
full state of the room, which is incorrect.

changelogs/server_server/newsfragments/3583.clarification

@@ -0,0 +1 @@
+Clarify that `GET /_matrix/federation/v1/event_auth/{roomId}/{eventId}` does *not* return the auth chain for the full state of the room.

data/api/server-server/event_auth.yaml

@@ -57,17 +57,13 @@ paths:
               auth_chain:
                 type: array
                 description: |-
-                  The full set of authorization events that make up the state of
-                  the room, and their authorization events, recursively. Note that
-                  events have a different format depending on the room version -
-                  check the [room version specification](/rooms) for precise event formats.
+                  The [PDUs](/server-server-api/#pdus) forming the auth chain
+                  of the given event. The event format varies depending on the
+                  room version - check the [room version specification](/rooms)
+                  for precise event formats.
                 items:
                   type: object
                   title: PDU
-                  description: |-
-                    The [PDUs](/server-server-api/#pdus) contained in the auth chain. The event format
-                    varies depending on the room version - check the [room version specification](/rooms)
-                    for precise event formats.
                   properties: []
                   example:
                     $ref: "examples/minimal_pdu.json"