Document iterators that iterate elements in a well-defined order

[CameronBieganek]

An old comment from Keno says that "iterators don't in general have a strong guarantee over their ordering". This seems like a large gap in our documentation. Looking at the docstring for UnitRange, there is no mention of the iteration order. So the following code is undefined:

x, y = 1:2

There's no guarantee that x == 1 and y == 2.

I propose that we update the docstrings for the various iterators in Base to make it clear which iterators have a well-defined iteration order. Perhaps we could use the word sequence to describe iterators with a well-defined iteration order.