Add some discussion about new lexers.

docs/source/whatsnew/pr/ipython-console-lexer.rst

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+New IPython Console Lexer
+-------------------------
+
+The IPython console lexer has been rewritten and now supports tracebacks
+and customized input/output prompts. An entire suite of lexers are now
+available at :module:`IPython.nbconvert.utils.lexers`. These include:
+
+    IPythonLexer
+    IPython3Lexer
+        Lexers for pure IPython (python + magic/shell commands)
+
+    IPythonPartialTracebackLexer
+    IPythonTracebackLexer
+        Supports 2.x and 3.x via the keyword `python3`.  The partial traceback
+        lexer reads everything but the Python code appearing in a traceback.
+        The full lexer combines the partial lexer with an IPython lexer.
+
+    IPythonConsoleLexer
+        A lexer for IPython console sessions, with support for tracebacks.
+        Supports 2.x and 3.x via the keyword `python3`.
+
+    IPyLexer
+        A friendly lexer which examines the first line of text and from it,
+        decides whether to use an IPython lexer or an IPython console lexer.
+        Supports 2.x and 3.x via the keyword `python3`.
+
+Previously, the :class:`IPythonConsoleLexer` class was available at
+:module:`IPython.sphinxext.ipython_console_hightlight`.  It was inserted
+into Pygments' list of available lexers under the name `ipython`.  It should
+be mentioned that this name is inaccurate.  An IPython console session
+is not the same as IPython code (which itself is a superset of the Python
+language).
+
+Now, the Sphinx extension inserts two console lexers into Pygment's list of
+available lexers.  Both are IPyLexer instances under the names:  `ipython` and
+`ipython3`. As mentioned above, these names are misleading, but they are kept
+for backwards compatibility and typical usage.  If a project needs to make
+Pygments aware of more than just the IPyLexer class, then one should not
+make the IPyLexer class available under the name `ipython` and use `ipy` or
+some other non-conflicting value.
+
+Code blocks such as::
+
+    .. code-block:: ipython
+
+       In [1]: 2**2
+       Out[1]: 4
+
+will continue to work as before, but now, they will also properly highlight
+tracebacks.  For pure IPython code, the same lexer will work::
+
+    .. code-block:: ipython
+
+       x = ''.join(map(str, range(10)))
+       !echo $x
+
+Since the first line of the block did not begin with a standard IPython console
+prompt, the entire block is assumed to be IPython code instead.