#699 order of plot properties, explanation for definition

[Karl Ratzsch]

In each plot-element, the sampling-range must be given after an eventual iteration (which makes a lot more sense anyway), e.g.

set sample 5; plot [0:10] for [i=0:9] [i:i+1] i w p

Attached patch fixes the documentation with regard to this.

It also adds an explanation for a "definition" as plot-element.