Remove excess markdown from govspeak helper in admin

https://www.pivotaltracker.com/story/show/62704998
Add examples of both full and shorthand markdown and explanations
changes in response to pull request feedback
Merge pull request #1179 from alphagov/tidy-table-chart-admin-markdown-helper

Remove excess markdown from govspeak helper in admin

https://www.pivotaltracker.com/story/show/62704998