docs(Dropdown): fix examples and update messages

[levithomason]

Fixes #1378. The confusion in that issue has come up several times. In response, this PR:

1. Updates old subcomponent API examples to use the shorthand props API where possible.
2. Updates the subcomponent API info message and adds it to all applicable examples.
3. Adds support for component context specific icon names, such as left dropdown.

Background

The Dropdown does not manage its own state when using the subcomponent API. Only the shorthand props API has access to the children (via a config object passed through props) to properly add event listeners and manage state.

Many of the doc site examples show the subcomponent API usage. Some are required to do this as we do not yet support the shorthand to create the variation displayed. Other examples use the subcomponent API simply because they are old. In both cases, very few subcomponent API examples give the proper information regarding this.

[codecov-io]

Codecov Report

Merging #1380 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1380   +/-   ##
=======================================
  Coverage   99.74%   99.74%           
=======================================
  Files         140      140           
  Lines        2346     2346           
=======================================
  Hits         2340     2340           
  Misses          6        6

Impacted Files	Coverage Δ
src/elements/Icon/Icon.js	100% <ø> (ø)	✅

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 82cdbe9...4427744. Read the comment docs.