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/flags.py

@@ -187,6 +187,8 @@ class MessageFlags(BaseFlags):
                Returns an iterator of ``(name, value)`` pairs. This allows it
                to be, for example, constructed as a dict or a list of pairs.
 
+    .. versionadded:: 1.3
+
     Attributes
     -----------
     value: :class:`int`