Improve media type encoding examples (3.1.1 port of 3864)

* Replace the outdated "model" terminology with "schema"
* Remove the outdated `text/plain` array example, which does not correlate with current OAS requirements
* Rather than replacing the `text/plain` example direclty, enhance the example of serializing `application/json` content in `application/x-www-form-urlencoeded` request bodies.