use versionchanged:: next

according to devguide

Author: minrk

Doc/library/asyncio-eventloop.rst

@@ -1051,7 +1051,7 @@ Watching file descriptors
 See also :ref:`Platform Support <asyncio-platform-support>` section
 for some limitations of these methods.
 
-.. versionchanged:: 3.15
+.. versionchanged:: next
 
    Added support for these methods to :class:`ProactorEventLoop`.
 

Doc/library/asyncio-platforms.rst

@@ -69,7 +69,7 @@ hardware (availability of `HPET
 <https://en.wikipedia.org/wiki/High_Precision_Event_Timer>`_) and on the
 Windows configuration.
 
-.. versionadded:: 3.15
+.. versionadded:: next
 
    Support for :meth:`loop.add_reader`, :meth:`loop.add_writer` added to :class:`ProactorEventLoop`.