Add a docstring to magics to fix the broken "%magic" magic.

IPython/frontend/prefilterfrontend.py

@@ -45,6 +45,8 b' def mk_system_call(system_call_function, command):'
     """
     def my_system_call(args):
         system_call_function("%s %s" % (command, args))
+
+    my_system_call.__doc__ = "Calls %s" % command
     return my_system_call
 
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