DOC: Update %matplotlib for bbox_inches

Added a couple of extra lines to the matplotlib magic doc string with
instructions on how to shut off the default bbox_inches='tight' behavior.

IPython/core/magics/pylab.py

@@ -57,9 +57,17 b' class PylabMagics(Magics):'
             
             In [2]: set_matplotlib_formats('pdf', 'svg')
 
-        See the docstring of `IPython.display.set_matplotlib_formats` and
+        The default for inline figures sets `bbox_inches` to 'tight'. This can
+        cause discrepancies between the displayed image and the identical
+        image created using `savefig`. This behavior can be disabled using the
+        `%config` magic::
+            
+            In [3]: %config InlineBackend.print_figure_kwargs = {'bbox_inches':None}
+
+        In addition, see the docstring of
+        `IPython.display.set_matplotlib_formats` and
         `IPython.display.set_matplotlib_close` for more information on
-        changing the behavior of the inline backend.
+        changing additional behaviors of the inline backend.
 
         Examples
         --------