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.
Good point. Looking back on this, it seems to make more sense to reference the name of the function using backticks.