Clarify ShorthandLinter spec structure

- Group example groups by whether they pertain to full rules or to a
  single property.
- Tidy example group names.

Related Story:
  http://www.pivotaltracker.com/story/show/43148129
  ("Teach scss-lint about missed shorthand case")

Change-Id: Iabb4d75cf9aaf8e8a85c66b1ccdd5a530afc2f68
Reviewed-on: https://gerrit.causes.com/17748
Reviewed-by: Alex Wallace <alex.wallace@causes.com>
Tested-by: Joe Lencioni <joe.lencioni@causes.com>