Add doc of HOMEBREW_ARCH

[dalance]

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I added a document of HOMEBREW_ARCH added by #6098.
My English is not native, please indicate any mistake.

[jonchang]

Please edit the source file located at Library/Homebrew/manpages/brew.1.md.erb. Then run brew man to generate manpages and markdown documentation and commit the result.

[dalance]

Thanks. I fixed it.

Comment addressed

[MikeMcQuaid]

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

[dalance]

Thank you for your review.