DOC: add behaviour change

doc/api/next_api_changes/behavior/22745-JMK.rst

@@ -0,0 +1,10 @@
+No need to specify renderer for get_tightbbox and get_window_extent
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The `.Artist.get_tightbbox` and `.Artist.get_window_extent` methods
+no longer require the *renderer* kwarg, saving users from having to
+querry it from ``fig.canvas.get_renderer``.  Currently, all Artists are
+aware of their parent figure (``artist.figure``), and if the *renderer*
+kwarg is not supplied these methods first check if there is a cached renderer
+from a previous draw and use that.  If there is no cahched renderer, then
+the methods will use ``fig.canvas.get_renderer()`` as a fallback.