Added clear_output changes

docs/source/whatsnew/pr/clear_output.rst

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+clear_output changes
+--------------------
+
+* There is no longer a 500ms delay when calling ``clear_output``.
+* The ability to clear stderr and stdout individually was removed.
+* A new wait flag that prevents ``clear_output`` from being executed until new 
+  output is available.  This eliminates animation flickering by allowing the 
+  user to double buffer the output.
+* The output div height is remembered when the ``wait=True`` flag is used.