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/guild.py

@@ -146,6 +146,8 @@ class Guild(Hashable):
         results to a specific language.
     discovery_splash: :class:`str`
         The guild's discovery splash.
+
+        .. versionadded:: 1.3
     """
 
     __slots__ = ('afk_timeout', 'afk_channel', '_members', '_channels', 'icon',