Merge pull request matplotlib#2948 from tacaswell/qt_repaint_doc

DOC : added missing doc changes from matplotlib#2844

Author: efiring

CHANGELOG

@@ -1,3 +1,7 @@
+2014-02-25 In backend_qt4agg changed from using update -> repaint under
+	   windows.  See comment in source near `self._priv_update` for
+	   longer explaination.
+
 2014-03-27 Added tests for pie ccw parameter. Removed pdf and svg images
            from tests for pie linewidth parameter.
 

doc/users/whats_new.rst

@@ -213,6 +213,9 @@ added. Furthermore, the the subplottool is now implemented as a modal
 dialog. It was previously a QMainWindow, leaving the SPT open if one closed the
 plotwindow.
 
+Interactive performance of the Qt4 backend has been dramatically improved
+under windows.
+
 Cairo backends
 ``````````````