Fixes and improvements for v1.3 documentation

* Add missing versionadded strings for v1.3
* Add missing versionchanged string for Message.edit
* Consistently use versionadded for attributes
* Consistently use versionchanged for parameters
* Use versionchanged for Bot.is_owner
* Fix references in v1.3 changelog
* Improve grammar in v1.3 changelog

discord/webhook.py

@@ -404,6 +404,9 @@ class Webhook:
         The webhook's ID
     type: :class:`WebhookType`
         The type of the webhook.
+
+        .. versionadded:: 1.3
+
     token: Optional[:class:`str`]
         The authentication token of the webhook. If this is ``None``
         then the webhook cannot be used to make requests.