Add acronym and abbreviation markdown guidance

* [x] Code review
* [x] Product review
* [x] Departments and policy checked copy

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* The style guide refers to “abbreviations and acronyms”, we should be consistent. All acronyms are abbreviations.
* Update the example to include multiple acronyms and some example content
* Explain how acronyms are found within content, and what happens when one is found. Include an `abbr` tag to illustrate the hover effect.
* Mention that the code itself isn’t displayed, it’s confusing when it initially disappears
* Include specifics about what should be listed (plurals, not possessives) and caveats to avoid (spaces in markdown)
* Improve typography with en dashes, smart quotes
  * Also, eg not e.g.

![screen shot 2014-08-20 at 12 43 27](https://cloud.githubusercontent.com/assets/319055/3980210/3df2b1ec-285f-11e4-808d-284c9589a35a.png)