Smart quotes and en dashes in markdown help

* http://smartquotesforsmartpeople.com
* Hyphens join words and separate syllables, en dashes denote a break
in a sentence – http://en.wikipedia.org/wiki/Dash
Add acronym and abbreviation markdown guidance

* The style guide refers to “abbreviations and acronyms”, we should be
consistent. All acronyms are abbreviations.
* Update the example to include some content containing an acronym
* 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
eg not e.g.

* Be consistent with our content style guide
Use href not data- to define a collapse target

* By including the href we get the default link styles and cursor
pointer we want
* Bootstrap defaults to using a target specified in the href of a link
Add more acronym guidance based on writing guide

* Based on
http://alphagov.github.io/inside-government-admin-guide/creating-documen
ts/markdown.html#acronyms
* Include specifics about what should be listed (plurals, not
possessives) and caveats to avoid (spaces in markdown)
* Add a second acronym to example so that markdown looks like a list
(the asterisk begins to make more sense in this context)
Remove extra guidance on acronyms

* Guidance around plurals and possessives confused users during
research session
* Copy above worked in explaining how acronym markdown behaves and how
it’s different to other types of markdown
* Extra guidance did not match out style guide (lists should have a
lead-in sentence and shouldn’t be capitalised)
Merge pull request #1675 from alphagov/AH-acronym-help

Add acronym and abbreviation markdown guidance