Small documentation fixes for astropy/modeling
tiny docfix: order matters for self.linear
 * a few more tiny doc tweaks
 * 1/2 -> 1/2.
 * rename "y" to "dummy" in gderiv
 * deriv method should be, not should return, None
 * added more docs about deriv method
 * linear after super.__init__
 * use super when __init__'ing