`flat` is not consistent with `Array.flat` about iter-ops

[vitaly-t]

That's true, Array.flat, as it turns out, doesn't really handle generic iterables like this library does, it only handles arrays. A string is an iterable object, that's why this library flattens it according to the depth/level specified.

A correct description should be that flat extends Array.flat, by applying flat to all iterable objects inside the list.

In general, this library does not separate arrays from generic iterables, it handles them in a unified way, so functionally it is correct, but I should update the docs about this.

[vitaly-t]

I have added the following idea, as a possible extension.