Document schema of PusherData in push gateway API (#1968)

Fixes: #910
Signed-off-by: Johannes Marbach <n0-0ne+github@mailbox.org>

changelogs/push_gateway/newsfragments/1968.clarification

@@ -0,0 +1 @@
+Document the schema of `PusherData`.

data/api/push-gateway/push_notifier.yaml

@@ -182,9 +182,15 @@ paths:
                             type: object
                             title: PusherData
                             description: |-
-                              A dictionary of additional pusher-specific data. For
+                              A dictionary of additional pusher-specific data. This
-                              'http' pushers, this is the data dictionary passed in at
+                              is the `data` dictionary passed in at
-                              pusher creation minus the `url` key.
+                              [pusher creation](/client-server-api/#post_matrixclientv3pushersset)
+                              minus the `url` key.
+                            properties:
+                              format:
+                                type: string
+                                description: |-
+                                  The format to use for sending notifications.
                           tweaks:
                             type: object
                             title: Tweaks