Add documentation for the startIndex argument of restArguments

[jgonggrijp]

Motivation: DefinitelyTyped/DefinitelyTyped#71151 (comment)

Preview of the added paragraph: https://cdn.statically.io/gh/jgonggrijp/underscore/document-restarguments-startindex/index.html#restArguments-startIndex

If anyone has suggestions for ways to simplify the explanation, I'm interested.

[jgonggrijp]

CC to @codejeffrey @Aurimas-stack @elkcityhazard @deining @colingm because you recently contributed or otherwise expressed interest. If one (or a few) of you would be willing to give this a quick review, I would highly appreciate it.

To review, just click on the "preview" link in the description and tell me whether you find the documentation clear enough. Bonus points if you can think of a way to simplify the explanation.

[jgonggrijp]

@GammaGames @Yahasana would one of you be willing to give this a review? Mentioning you because you expressed willingness to be mentioned over at Backbone.

[colingm]

@jgonggrijp one of the challenges I see with the documentation for this function in general (the original example too, not just your additions), is the lack of showing the output. In various other function docs we see logs to a mock console but in this one we are just shown that the rest get called to "sendConsolations". I think it would go a long way to show with some mock console output a log statement of like "Sorry Sneezy!", "Sorry Bashful!", etc. and then to show that with the start index it is actually "Sorry Happy!" etc.

[jgonggrijp]

@colingm great suggestion, thanks!