Add whatsnew entry for unicode completion

docs/source/whatsnew/pr/latex-unicode.rst

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+Typing unicode identifiers
+--------------------------
+
+.. image:: /_images/unicode_completion.png
+
+Complex expressions can be much cleaner when written with a wider choice of
+characters. Python 3 allows unicode identifiers, and IPython 3 makes it easier
+to type those, using a feature from Julia. Type a backslash followed by a LaTeX
+style short name, such as ``\alpha``. Press tab, and it will turn into α.