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.

diff --cc src/doc/tutorial.md
Simple merge