doc: add range iterators in the for loop section

[adrientetar]

No description provided.

[brendanzab]

Is there any reason you decided to put it above the part about for being sugar? Because that applies to range as well. I'm also not convinced that range_inclusive needs to be mentioned.

[adrientetar]

1. The way I see it, the point of saying that for is sugar here is precisely to point out that it can go through any array as iterator which isn't common on today's most used programming languages (afaik).
2. I like to use it for semantic purposes (eq. to >= or <= operators in C). A good compromise would be removing the example but still mentioning it in the parenthesis (through the example shows that it has to be imported, useful for the newbie willing to get started fast).

[huonw]

@thestinger

[thestinger]

I think it would make more sense to mention range, range_inclusive, count and unfoldr in a separate section, after the for loop one. I'm okay with this landing as is though, I'll just play with it a bit later.