https://docs.brew.sh/Manpage#cask-command-options-cask
brew cask info github
should work.
--desc is not listed as a valid option for cask.
To specifically check a field in the JSON file /api/cask/github.json (JSON API), use the info command with --json flag and use jq to get the values of a particular field.
brew info --json=v1 github | jq .desc