Tutorial doc

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.