Be more clear about what this lambda expression does
Give more explanation when introducing closures
Demonstrate accessing external variable in first example
Some more closure changes
Fix typo
Remove extra semicolons
Suggested revisions to PR 13676.

Most important: distinguish function decl sugar for omitting `-> ()`
from type inference on closures.  I also tried to add a couple more
examples to further emphasize this distinction.  Note that this sugar
(of omitting `-> ()`) is actually already briefly mentioned in an
earlier section, so it is a little tricky deciding whether to put more
material here, or to move it up to the previous section.

Other drive-by fixes:

 * Fix the line length of the code blocks to fit in the width provided
   in the rendered HTML

 * Some minor revisions to wording (e.g. try to clarify in some cases
   where a type mismatch is arising).
Merge pull request #1 from pnkfelix/clarify-omit-unit-vs-inference

Suggested revisions to PR 13676.
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.