Fix the error in Doc: How to Open a Homebrew Pull Request

[ChenZhongPu]

The Doc How to Open a Homebrew Pull Request has some errors, which may mislead user contributors. To be specific,

• --build-from-source is for formula only.
• brew test is for formula only.

This pull request is to separate the tests for formulae/cask related changes.

• 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.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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[apainintheneck]

You're right that the current documentation is a bit confusing and has errors. If possible, I think we should avoid having headings in bulleted lists though since that can make things a bit harder to follow and it seems like we're trying to avoid doing that in this doc.

I wonder if it would make sense to somehow split up the ## Create your pull request from a new branch heading so that we can provide specific formula, cask and main repository testing/auditing/linting instructions.

[apainintheneck]

Question: Why is HOMEBREW_NO_AUTO_UPDATE=1 needed now and only for cask-related changes?

[ChenZhongPu]

It is to stop brew updating itself while installing a new cask or formula. This option makes sense for both cask and formula.

[apainintheneck]

So the worry is that new changes might get pulled down from the remote tap that create conflicts with local formula/cask changes?