Changes after editor review; better line wrapping.

As I did with the [Orchestration user guide](https://github.com/rackspace/php-opencloud/blob/master/docs/userguide/Orchestration/USERGUIDE.md), I asked our editor, Kelly Holcomb, to review the Networking user guide. This PR contains edits based on her review + changes to reasonably restrict lines to 80 character width.