Miscellaneous changes to Rust Guide

I was reading through the Rust Guide (hopefully looking to learn some Rust), and I figured it would be a good idea to open a pull request with some of the errata I noticed along the way. Most of the changes are pretty mundane, but there are a couple that might raise a bit of discussion.

### Punctuation outside of 'key term' quotes

This is something that was inconsistent in the Guide. While the convention in American English is to place punctuation immediately following a quotation mark *inside* the quotation mark, it seems strange to do this with 'key terms', considering they are not a true quotation.

### Changed comment placement in 17.2 code block

This is what the code block in [17.2 — Ownership, borrowing, and lifetimes](http://doc.rust-lang.org/guide.html#ownership,-borrowing,-and-lifetimes) looks like in fullscreened Safari 8:

![screen shot 2014-12-13 at 2 48 47 pm](https://cloud.githubusercontent.com/assets/2434728/5425704/2fff3bf0-82d7-11e4-8c8f-d594acde8937.png)

Some of the comments extend *just* too far, causing them to bleed into the next line, so I moved a few of them above the relevant lines of code to avoid this.
this isn't a Rust-specific concept and therefore wouldn't be part of the 'punctuation outside' convention we're talking about here.