Add info about new legend events

[etpinard]

from plotly/plotly.js#2581 (comment)

[jacobq]

In particular, I'd like to see examples / recommendations regarding how to prevent firing the click handlers in the case of doubleclick events. (IIRC, this problem also occurs when dealing directly with the DOM, e.g. https://output.jsbin.com/nikimir/3)

[michaelbabyn]

closed by #1002