auto merge of #13676 : mdinger/rust/tutorial_doc, r=pnkfelix
Improve tutorial discussion of closures, e.g. with respect to type inference and variable capture.
Fix #13621
---- original description follows
I'd like this pulled to master if possible but if not I'd appreciate comments on what I need to change. I found the closures difficult to understand as they were so I tried to explain it so I would've had an easier time understanding it. I think it's better at least, somewhat.
I don't know that everyone liked the `-> ()` I included but I thought explicit is best to aid understanding. I thought it was much harder to understand than it should have been.
[EDIT] - Clicked too early.
This doesn't `make check` without errors on my Xubuntu on Virtualbox machine. Not sure why. I don't think I changed anything problematic. I'll try `make check` on master tomorrow.
Opened https://github.com/mozilla/rust/issues/13621 regarding this.