docs: document HOMEBREW_INSTALL_FROM_API usage in installation

[kidonng]

• 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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This feature is added in Homebrew/install#628, but I forgot to document it here.

[MikeMcQuaid]

Looks good! Some safety tweaks.

[MikeMcQuaid]

Thanks again @kidonng!

[Rylan12]

It's probably worth clarifying that you also need to set HOMEBREW_INSTALL_FROM_API in your shell profile file so it persists in new sessions. Otherwise, the next time you try to brew install in a new session, it will try to tap homebrew/core again then.

[kidonng]

The "Git Remote Mirroring" section above does not emphasize this point, I'm just following suite. The install script does mention setting the remotes, perhaps this should be resolved there.