Clarify that usage of event_id_only is not mandatory

Fixes: #1026
Signed-off-by: Johannes Marbach <n0-0ne+github@mailbox.org>

changelogs/client_server/newsfragments/2255.clarification

@@ -0,0 +1 @@
+Usage of `event_id_only` is not mandatory.

data/api/client-server/pusher.yaml

@@ -230,11 +230,14 @@ paths:
                     format:
                       type: string
                       description: |-
-                        The format to send notifications in to Push Gateways if the
+                        The format in which to send notifications to the push gateway
-                        `kind` is `http`. The details about what fields the
+                        if the `kind` is `http`. The details about what fields the
-                        homeserver should send to the push gateway are defined in the
+                        homeserver should include are defined in the
-                        [Push Gateway Specification](/push-gateway-api/). Currently the only format
+                        [Push Gateway Specification](/push-gateway-api/). If omitted,
-                        available is 'event_id_only'.
+                        the server is expected to populate all of the event-related fields
+                        specified in [`/_matrix/push/v1/notify`](/push-gateway-api/#post_matrixpushv1notify).
+                      enum:
+                        - "event_id_only"
                 append:
                   type: boolean
                   description: |-