Update control-flow.markdown - for statement section

[PureFox48]

Fixes #1176.

[RobLoach]

Good call to mention that variable scope it kept for the function.

It may make more sense to document this in the Functions section, as this seems more like a property of closures, rather than loops and control flow.

[PureFox48]

Well, the Closures section already makes clear what it means to capture a variable.

This PR is peculiar to the for statement where it looks like there’s only one variable for the entire loop when in fact there’s a separate instance for each iteration. Although this only really manifests itself when there’s a closure involved, I still think that the Control Flow section is the best place to put it.