ARROW-13018: [C++][Docs] Use consistent terminology for nulls (min_count) in scalar aggregate kernels
ARROW-13018: [C++][Docs] Use consistent terminology for nulls (min_count) in scalar aggregate kernels

Closes #10490 from jorisvandenbossche/ARROW-13018

Authored-by: Joris Van den Bossche <36c737f86d71d8ee1ddeb1e5c83102ee5de8f97c@gmail.com>
Signed-off-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>

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