Patch new cask documentation

[tomy0000000]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
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• Have you successfully run brew tests with your changes locally?

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While adding a new cask (Homebrew/homebrew-cask#164152), the testing command failed when HOMEBREW_NO_INSTALL_FROM_API env is not set. This is documented in the section for adding new formulae but not in the section for new casks.

This PR patches the commands.

[MikeMcQuaid]

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @tomy0000000!