Document x-enum-varnames and x-enum-descriptions as vendor extensions

[jimschubert]

Description

Add x-enum-varnames and x-enum-descriptions to documentation.

Related issues/PRs

Introduced via #1752, x-enum-descriptions should be added to the new doc site which will be introduced by #1770.

While reviewing #1752, I also noticed x-enum-varnames which is not accounted for in the existing docs that I had migrated to the docusaurus site. Once #1770 is merged, these should be documented in docs/templating.md.

[jmini]

On gitter @wing328 pointed to:
https://github.com/OpenAPITools/openapi-generator/wiki/Vendor-Extensions

Should I update this page or wait for #1770 to be merged?

---

In #1693 (comment) there is an example to demonstrate how both x-enum-varnames and x-enum-descriptions can be used.

[jimschubert]

@jmini please wait for #1770, as I've already migrated the docs and will edit them to link to the new locations after the site is up and running.

[jmini]

I will update https://github.com/OpenAPITools/openapi-generator/blob/master/docs/templating.md#extensions

[jmini]

PR: #2010