Add docs on spec language support

[rly]

Motivation

Fix #1068

Checklist

• Did you update CHANGELOG.md with your changes?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed our Contributing Guide?
• Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.53%. Comparing base (048b9b8) to head (3b30717).
Report is 55 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1069   +/-   ##
=======================================
  Coverage   88.53%   88.53%           
=======================================
  Files          45       45           
  Lines        9605     9605           
  Branches     2732     2732           
=======================================
  Hits         8504     8504           
  Misses        778      778           
  Partials      323      323           

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