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

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.