give better description of [line] behavior in help file

The help file for [line] leaves it unstated whether the total ramp duration is fudged to quantize to the grain rate, or whether the grain rate is fudged to accommodate a precise total ramp duration. (It's the latter, with a final message sent below the grain rate when the grain rate doesn't divide evenly into the total ramp duration.)

Make this behavior explicit in the help file. Preferably without using the word "fudge."