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GH-35740: Add documentation for list arrays' values property (#35865)

Just docs.

I'm a little shaky on my understanding of exactly what's going on with FixedSizeListArray.values, and its behavior with nulls, so that wording might deserve a careful read.
* Closes: #35740

Lead-authored-by: Spencer Nelson <spencer@b612foundation.org>
Co-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Co-authored-by: Alenka Frim <AlenkaF@users.noreply.github.com>
Signed-off-by: AlenkaF <frim.alenka@gmail.com>