docs: add body options for snapshot api

docs/sources/http_api/snapshot.md

@@ -52,6 +52,15 @@ parent = "http_api"
       "expires": 3600
     }
 
+JSON Body schema:
+
+- **dashboard** – Required. The complete dashboard model.
+- **name** – Optional. snapshot name
+- **expires** - Optional. When the snapshot should expire in seconds. 3600 is 1 hour, 86400 is 1 day. Default is never to expire. 
+- **external** - Optional. Save the snapshot on an external server rather than locally. Default is `false`.
+- **key** - Optional. Define the unique key. Required if **external** is `true`.
+- **deleteKey** - Optional. Unique key used to delete the snapshot. It is different from the **key** so that only the creator can delete the snapshot. Required if **external** is `true`.
+
 **Example Response**:
 
     HTTP/1.1 200