[Rd] Class generator functions for reference classes

[John Chambers]

As of rev. 61035 in r-devel, setRefClass() now returns a generator 
function, as setClass() has done since 2.15.0.

The convenient style is now:
   mEdit <- setRefClass("mEdit",......)
   xx <- mEdit(data = xMat)

instead of
   xx <- mEdit$new(data = xMat)

The returned object still has fields and methods accessible as before.

See the "Value" and "Reference Class Generators" sections of 
?ReferenceClasses for details.

Thanks to Romain François for suggesting this.

John