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ARROW-12242: [Python][Doc] Tweak nightly build instructions

Nudge pip towards downloading a binary build, even if slightly outdated, rather than build from source.

Closes #9921 from pitrou/ARROW-12242-py-doc-nightlies

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>