Simpler if statement and added line to the docs

IPython/core/magics/pylab.py

@@ -42,7 +42,7 b' class PylabMagics(Magics):'
     @skip_doctest
     @line_magic
     @magic_arguments.magic_arguments()
-    @magic_arguments.argument('-l', '--list', default=None, action='store_true',
+    @magic_arguments.argument('-l', '--list', default=False, action='store_true',
                               help='Show available matplotlib backends')
     @magic_gui_arg
     def matplotlib(self, line=''):
@@ -85,11 +85,15 b' class PylabMagics(Magics):'
         But you can explicitly request a different GUI backend::
 
             In [3]: %matplotlib qt
-        """
 
-        args = magic_arguments.parse_argstring(self.matplotlib, line)
+        You can list the available backends using the -l/--list option
 
-        if args.list is not None:
+           In [4]: %matplotlib --list
+           Available matplotlib backends: ['osx', 'qt4', 'qt5', 'gtk3', 'notebook', 'wx', 'qt', 'nbagg',
+            'gtk', 'tk', 'inline']
+        """
+        args = magic_arguments.parse_argstring(self.matplotlib, line)
+        if args.list:
             backends_list = list(backends.keys())
             print("Available matplotlib backends: %s" % backends_list)
         else: