Shorten the introduction to compound models per @astrofrog's comment here: https://github.com/astropy/astropy/pull/3231#issuecomment-68179595  I agree it was a bit overload.  The main docs still start out by explaining compound model class creation first, since I think it's helpful for understanding how the sausage is made.  But it's made more clear that this is 'advanced' usage.