Document for-loop in tutorial section on loops
 Explain container iteration in the loop tutorial

 As suggested by @pnkfelix in #12161, this extends the examples by a for-loop
 that iterates over a string as an example for container iteration.
auto merge of #12161 : aepsil0n/rust/docs/for-loop, r=alexcrichton

I just started learning Rust and the absence of an explanation of the for-loop in the beginning really bugged me about the tutorial. Hence I simply added these lines, where I would have expected them. I know that there is something later on in the section on traits. However, this simple iteration scheme feels like something that you should be aware of right away.