Documenting state class naming convention.

Add section documenting agreed-upon state class naming convention.

Status: **Ready to merge**

Reviewers: @jeffkamo @kpeatt 
Ticket: N/A
Linked PRs: N/A

## Changes
- Add section describing how to name classes to style component states
    - Document the basic convention with examples
    - Document the `has-state` alternate version
    - Document how and when to style on ARIA state attributes instead of classes
- Shorten “modifier elements” in the property order list to just “modifiers” since modifiers aren’t for stand-alone elements

## Research resources
- For a list of ARIA states, see [this list](http://www.w3.org/TR/wai-aria/states_and_properties#global_states_header) (look for the properties with “state” at the end)