set `__doc__` for aliases

fixes %magic output summary

IPython/core/alias.py

@@ -131,6 +131,7 b' class Alias(object):'
         self.shell = shell
         self.name = name
         self.cmd = cmd
+        self.__doc__ = "Alias for `!{}`".format(cmd)
         self.nargs = self.validate()
 
     def validate(self):