Updated docstring for agg backend and gui

Updated the list of possible values for gui in the
docstring, according to my previous commit.

IPython/core/pylabtools.py

@@ -246,7 +246,7 b' def find_gui_and_backend(gui=None, gui_select=None):'
     Parameters
     ----------
     gui : str
-        Can be one of ('tk','gtk','wx','qt','qt4','inline').
+        Can be one of ('tk','gtk','wx','qt','qt4','inline','agg').
     gui_select : str
         Can be one of ('tk','gtk','wx','qt','qt4','inline').
         This is any gui already selected by the shell.
@@ -254,7 +254,7 b' def find_gui_and_backend(gui=None, gui_select=None):'
     Returns
     -------
     A tuple of (gui, backend) where backend is one of ('TkAgg','GTKAgg',
-    'WXAgg','Qt4Agg','module://ipykernel.pylab.backend_inline').
+    'WXAgg','Qt4Agg','module://ipykernel.pylab.backend_inline','agg').
     """
 
     import matplotlib