paper-slider needs documentation of 'step' property

[kenmoore]

The online doc for snaps says "If true, the slider thumb snaps to tick marks evenly spaced based on the step property value." There's no explanation of how the step property works or how to apply it.

After some experimentation I figured it out, but docs would help greatly.

[frankiefu]

paper-slider extends from core-range and the step property is documented there.